lua, 59 bytes

Result

AC

Duration

753ms

Code [DL]

while''do a,b=io.read(4,1,3)print(a:match('(.)%1 ')or b)end

stdin

854 563
866 667
244 649
165 664
611 356
215 124
722 696
885 851
577 478
712 334
624 125
323 484
943 736
319 915
484 441
632 334
338 681
655 597
645 175
485 545
173 975
314 449
411 664
188 758
437 731
772 683
574 454
422 248
143 886
787 875
588 887
411 221

stdout

5
6
4
6
1
1
2
8
7
3
1
4
7
9
4
3
6
5
1
5
9
4
1
8
7
6
4
2
8
8
8
1

stderr

/usr/bin/lua5.3: /volume/CODE:1: attempt to index a nil value (global 'a')
stack traceback:
	/volume/CODE:1: in main chunk
	[C]: in ?

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7fff507dfc10 /* 7 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x56187c28cd30 /* 7 vars */ <unfinished ...>
12    execve("/usr/bin/lua5.3", ["/usr/bin/lua5.3", "/volume/CODE"], 0x56187c28cd40 /* 7 vars */ <unfinished ...>
11    <... execve resumed>)             = 0
12    <... execve resumed>)             = 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    +++ exited with 1 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=1, si_utime=0, si_stime=0} ---
10    +++ exited with 1 +++