#!/usr/bin/env perl
use strict;
my $n=$ARGV[0];
my $result=1;

if ($n<0 || int ($n)!=$n){print "ERROR \n"; die;}

print "!$n=";

while ($n>0)
  {
    $result=$result*$n;
    $n=$n-1;
  }
print "$result\n";