<?php
    $x = [];
    while ($line = fgets(STDIN)) {
        $x[] = explode(" ", trim($line)); //各行の値を変数xに代入、保管
    }
    
    $mame = $x[0][0] * $x[0][1];
    
    $total = 0;
    foreach($x[1] as $v) {
        $total += $v;
    }
    
    $diff = $mame - $total;
    if ($diff >= 0) {
        echo $diff;
    } else {
        echo -1;
    }