#!/usr/bin/perl -lw

sub c {
  my $r = $i = 0;
  map {$r += index(' abcdefghijklmnopqrstuvwxyz',$_) * (26 ** $i++);} reverse split (//, lc(shift));
  $r;
}

$s = c('McD');
$t = c('JAPH');
$d = $t-$s;

for($i=0, $c='McD'; $i<$d; $i++, $c++) {}

print $c;