<?PHP
//   cd dot/PHP
//   PHP try.php

list($nKazu,$nFukuro,$nMember)=explode(" ",trim(fgets(STDIN)));
$arr_olds=explode(" ",trim(fgets(STDIN)));

$sum_old=array_sum($arr_olds);
$ntotal=$nKazu*$nFukuro;

if($sum_old>$ntotal){
	echo "-1".PHP_EOL;
	exit;
}else{
	$nAns=$ntotal-$sum_old;
	echo $nAns.PHP_EOL;
}