#!/usr/bin/perl

use warnings;
use strict;

my ($k, $n, $f) = split / /, <>;
my $year = <>;
chomp $year;
my @array = split / /, $year;
my $total = 0;

for(my $i = 0; $i < $f; $i++) {
	$total += $array[$i]; 
}

my $ans = $k * $n - $total;

if($ans < 0) {
	print "-1\n";
} else {
	print "$ans\n";
}

exit;