<?php

$input = explode(" ", trim(fgets(STDIN)));
$A = explode(" ", trim(fgets(STDIN)));

$K = $input[0];
$N = $input[1];
$F = $input[2];

//豆の数
$number = $K * $N;
//すべての年齢の和
$wa = 0;
for($i = 0; $i < $F; $i++){
	$wa += $A[$i];
}
$diff = $number - $wa;
if($diff >= 0){
	echo $diff, PHP_EOL;
}else if($diff < 0){
	echo -1, PHP_EOL;
}