#A common security method used for online banking is to ask the user for three random characters from a passcode. For example, if the passcode was 531278, they may asked for the 2nd, 3rd, and 5th characters; the expected reply would be: 317.
#The text file, keylog.txt, contains fifty successful login attempts.
#Given that the three characters are always asked for in order, analyse the file so as to determine the secret passcode of unknown length.
# assumptions - every digit that is in the passcode is in the file
# no digit is repeated in the sequence
openmy$fh,'keylog.txt'ordie"no open keylog.txt: $!\n";