perl, 23 bytes

Result

WA

Duration

741ms

Code [DL]

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

stdin

597 459
137 338
419 669
918 232
211 131
789 949
588 149
429 923
423 631
927 922
143 435
835 558
445 455
177 668
379 767
664 452
677 578
516 918
933 936
128 961
755 992
599 986
442 245
988 887
166 664
694 149
898 885
762 883
152 559
155 786
872 743
457 241

stdout

3661989922557747733592866885555

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffef61c7a60 /* 7 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x5609fc02ed30 /* 7 vars */) = 0
12    execve("/usr/bin/perl", ["/usr/bin/perl", "/volume/CODE"], 0x5609fc02ed40 /* 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    +++ 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 +++