#!/usr/bin/env perl

use bigint;

my ($b1, $b2, $b3) = split / /, <>; chomp $b3;
if ($b2 - $b1 == 0) {
  print $b1;
} else {
  my $r = ($b3 - $b2) / ($b2 - $b1);
  my $d = $b3 - $r * $b2;
  print $r * $b3 + $d;
}