<?php
list($k,$n,$f) = explode(" ",trim(fgets(STDIN)));
$a = explode(" ",trim(fgets(STDIN)));
$mame = $k * $n;
$w = array_sum($a);
$r = $mame - $w;
if($r >= 0){
echo $r."\n";
}else{
echo "-1\n";
}