"use strict";
function Main(input) {
    let net;
    let qty;
    let fam;
    [net, qty, fam] = input.split("\n")[0].split(" ").map(i => Number(i));
    const ages = input.split("\n")[1].split(" ").map(i => Number(i)).reduce((accum, current) => accum + current);
    const beans = net * qty;
    if (beans < ages) {
        console.log(-1);
    }
    else {
        console.log(beans - ages);
    }
}
//テストの入力を受け取る
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
//Main("5 5 3\n3 10 5");
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