perl, 23 bytes

Result

AC

Duration

748ms

Code [DL]

/.(.) ?\1/,print$1for<>

stdin

211 612
649 453
822 415
622 227
655 581
266 613
143 432
455 452
577 447
116 115
563 643
463 735
942 428
242 348
185 569
544 132
185 221
661 113
436 763
151 955
595 714
562 269
299 551
198 993
742 246
373 425
245 551
868 951
238 998
711 118
663 873
151 169

stdout

11225665711111542115529922559111

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffcafc548b0 /* 7 vars */) = 0
12    execve("/usr/bin/perl", ["/usr/bin/perl", "/volume/CODE"], 0x56338e26bd40 /* 7 vars */ <unfinished ...>
11    execve("/bin/cat", ["cat", "-"], 0x56338e26bd30 /* 7 vars */ <unfinished ...>
12    <... execve resumed>)             = 0
11    <... 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 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} ---
10    +++ exited with 0 +++