ruby, 27 bytes

Result

WA

Duration

954ms

Code [DL]

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

stdin

943 881
535 335
911 112
422 259
425 561
497 398
592 688
284 945
123 387
155 994
775 775
684 445
922 281
612 726
547 368
566 665
134 344
997 336
988 949
545 895
399 597
765 562
932 347
841 148
273 877
966 116
654 648
398 883
977 874
655 341
551 577
513 133

stdout

83125

stderr

/volume/CODE:2:in `[]': no implicit conversion from nil to integer (TypeError)
	from /volume/CODE:2:in `<main>'

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffc161576a0 /* 9 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x557a705efd40 /* 9 vars */) = 0
12    execve("/usr/local/ruby-2.7.1/bin/ruby", ["/usr/local/ruby-2.7.1/bin/ruby", "/volume/CODE"], 0x557a705efd60 /* 9 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 1 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=1, si_utime=5, si_stime=1} ---
10    +++ exited with 1 +++