<?php

    fscanf(STDIN,"%d %d %d",$kosu,$fukuro,$family);
    $age = explode(" ",trim(fgets(STDIN)));
    
    $fukuro_sum = $kosu * $fukuro;
    $age_sum = array_sum($age);
    
    $ans = $fukuro_sum - $age_sum;
    if ( $ans >= 0) {
        echo $ans;
    } else {
        echo "-1";
    }

?>