golang, 107 bytes

Result

AC

Duration

1803ms

Code [DL]

package main
import f"fmt"
func main(){for{a:=""
b:=a
f.Scan(&a,&b)
if a[1]==a[2]{b=a[2:]}
f.Print(b[:1])}}

stdin

363 116
855 556
996 991
678 479
677 998
755 156
845 776
466 113
412 559
326 227
878 285
151 118
599 992
112 816
546 647
735 685
722 723
881 518
971 179
955 422
221 964
951 865
525 558
477 491
466 691
546 427
837 372
962 616
528 282
482 273
922 249
371 155

stdout

15947576522198662515985764362221

stderr

panic: runtime error: index out of range [1] with length 0

goroutine 1 [running]:
main.main()
	/tmp/code.go:6 +0x1e3

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffebff769c0 /* 7 vars */) = 0
11    execve("/usr/bin/realpath", ["realpath", "/volume/CODE"], 0x55bff2e78d18 /* 7 vars */) = 0
11    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
12    execve("/bin/ln", ["ln", "-sf", "/volume/CODE", "/tmp/code.go"], 0x55bff2e78d38 /* 7 vars */) = 0
12    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
13    execve("/usr/bin/go", ["/usr/bin/go", "build", "-o", "/tmp/code", "/tmp/code.go"], 0x55bff41b7028 /* 7 vars */) = 0
19    execve("/usr/lib/go/pkg/tool/linux_amd64/compile", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-V=full"], 0xc000118750 /* 25 vars */) = 0
20    execve("/usr/lib/go/pkg/tool/linux_amd64/compile", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-V=full"], 0xc00012ed00 /* 25 vars */) = 0
22    +++ exited with 0 +++
23    +++ exited with 0 +++
24    +++ exited with 0 +++
21    +++ exited with 0 +++
19    +++ exited with 0 +++
16    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=19, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
25    execve("/usr/lib/go/pkg/tool/linux_amd64/asm", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-V=full"], 0xc0001188f0 /* 25 vars */) = 0
28    +++ exited with 0 +++
29    +++ exited with 0 +++
27    +++ exited with 0 +++
26    +++ exited with 0 +++
20    +++ exited with 0 +++
13    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=20, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
32    execve("/usr/lib/go/pkg/tool/linux_amd64/asm", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-V=full"], 0xc000118a90 /* 25 vars */) = 0
33    +++ exited with 0 +++
34    +++ exited with 0 +++
35    +++ exited with 0 +++
36    +++ exited with 0 +++
25    +++ exited with 0 +++
13    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=25, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
38    +++ exited with 0 +++
40    +++ exited with 0 +++
39    +++ exited with 0 +++
37    +++ exited with 0 +++
32    +++ exited with 0 +++
16    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=32, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
41    execve("/usr/lib/go/pkg/tool/linux_amd64/compile", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-o", "/tmp/go-build215194601/b001/_pkg"..., "-trimpath", "/tmp/go-build215194601/b001=>", "-p", "main", "-complete", "-buildid", "OhYca_T7eTS7E566erXW/OhYca_T7eTS"..., "-goversion", "go1.13.10", "-D", "_/tmp", "-importcfg", "/tmp/go-build215194601/b001/impo"..., "-pack", "-c=2", "/tmp/code.go"], 0xc000484410 /* 25 vars */) = 0
45    +++ exited with 0 +++
44    +++ exited with 0 +++
43    +++ exited with 0 +++
42    +++ exited with 0 +++
41    +++ exited with 0 +++
31    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=41, si_uid=0, si_status=0, si_utime=2, si_stime=0} ---
46    execve("/usr/lib/go/pkg/tool/linux_amd64/link", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-V=full"], 0xc000362340 /* 25 vars */) = 0
50    +++ exited with 0 +++
48    +++ exited with 0 +++
49    +++ exited with 0 +++
47    +++ exited with 0 +++
46    +++ exited with 0 +++
31    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=46, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
51    execve("/usr/lib/go/pkg/tool/linux_amd64/link", ["/usr/lib/go/pkg/tool/linux_amd64"..., "-o", "/tmp/go-build215194601/b001/exe/"..., "-importcfg", "/tmp/go-build215194601/b001/impo"..., "-buildmode=exe", "-buildid=zOUTzo7IDPSt95cySA1z/Oh"..., "-extld=gcc", "/tmp/go-build215194601/b001/_pkg"...], 0xc000362750 /* 25 vars */) = 0
55    +++ exited with 0 +++
54    +++ exited with 0 +++
53    +++ exited with 0 +++
52    +++ exited with 0 +++
51    +++ exited with 0 +++
18    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=51, si_uid=0, si_status=0, si_utime=18, si_stime=4} ---
31    +++ exited with 0 +++
30    +++ exited with 0 +++
18    +++ exited with 0 +++
17    +++ exited with 0 +++
16    +++ exited with 0 +++
15    +++ exited with 0 +++
14    +++ exited with 0 +++
13    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13, si_uid=0, si_status=0, si_utime=3, si_stime=11} ---
56    execve("/bin/cat", ["cat", "-"], 0x55bff2e78d08 /* 7 vars */) = 0
56    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=56, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
57    execve("/tmp/code", ["/tmp/code"], 0x55bff2e78cf0 /* 7 vars */) = 0
59    +++ exited with 2 +++
58    +++ exited with 2 +++
60    +++ exited with 2 +++
57    +++ exited with 2 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=57, si_uid=0, si_status=2, si_utime=0, si_stime=0} ---
61    execve("/bin/rm", ["rm", "/tmp/code.go", "/tmp/code"], 0x55bff2e78cf0 /* 7 vars */) = 0
61    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=61, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
10    +++ exited with 0 +++