perl, 25 bytes

Result

WA

Duration

847ms

Code [DL]

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

stdin

697 298
265 654
855 558
218 882
949 972
911 588
168 636
718 846
129 626
734 358
822 779
424 942
366 952
614 994
549 941
424 551
129 998
922 259
425 926
499 796
355 556
215 114
369 664
633 312
854 147
826 355
611 612
644 886
195 592
126 516
341 414
586 733

stdout

589168662269959229516335145543

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffef03a12a0 /* 7 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x5634996fed30 /* 7 vars */ <unfinished ...>
12    execve("/usr/bin/perl", ["/usr/bin/perl", "/volume/CODE"], 0x5634996fed40 /* 7 vars */ <unfinished ...>
11    <... execve resumed>)             = 0
12    <... 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 +++