#include <iostream>
#include <string>
using namespace std;
int main() {
    int a, b, c, d, n, i, sum = 0;
    cin >> a >> b >> n;
    c = a * b;
    for (i=0; i<n; i++) {
        cin >> d;
        sum += d;
    }
    c -= sum;
    if (c < 0) {
        cout << -1 << endl;
    } else {
        cout << c << endl;
    }
    return 0;
}