ruby, 29 bytes

Result

AC

Duration

1325ms

Code [DL]

#!ruby -p
$_=$_[~/(.)\1 /||4]

stdin

411 188
375 575
185 559
788 998
238 326
486 558
545 467
744 441
741 112
866 645
158 283
593 633
588 587
869 967
337 375
674 749
648 877
426 837
286 125
866 664
692 194
837 634
566 893
433 939
771 323
877 365
276 697
379 776
127 259
433 771
698 227
761 665

stdout

15583544162689378816166337672326

stderr

strace

11    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7fffd295e8c0 /* 9 vars */) = 0
12    execve("/bin/cat", ["cat", "-"], 0x55b784b2ad40 /* 9 vars */) = 0
12    +++ exited with 0 +++
11    --- 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/local/ruby-2.7.1/bin/ruby", ["/usr/local/ruby-2.7.1/bin/ruby", "/volume/CODE"], 0x55b784b2ad60 /* 9 vars */) = 0
13    +++ exited with 0 +++
11    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13, si_uid=0, si_status=0, si_utime=6, si_stime=2} ---
11    +++ exited with 0 +++