ruby, 27 bytes

Result

WA

Duration

907ms

Code [DL]

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

stdin

866 664
346 565
674 479
458 295
227 761
277 226
115 662
285 564
116 611
288 758
439 392
229 723
383 862
591 918
719 996
884 176
956 351
822 827
636 262
648 881
756 476
588 881
328 223
922 268
221 556
933 446
769 677
477 354
233 634
366 645
543 448
719 358

stdout

6

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"], 0x7fff8d056530 /* 9 vars */) = 0
12    execve("/usr/local/ruby-2.7.1/bin/ruby", ["/usr/local/ruby-2.7.1/bin/ruby", "/volume/CODE"], 0x562b81747d60 /* 9 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x562b81747d40 /* 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=6, si_stime=4} ---
10    +++ exited with 1 +++