#include <bits/stdc++.h>
using namespace std;

int main()
{
    int k, n, f;
    cin >> k >> n >> f;
    int total = k * n;
    for (int i = 0; i < f; i++) {
        int a;
        cin >> a;
        total -= a;
    }

    if (total < 0) {
        cout << -1;
    } else {
        cout << total;
    }
    return 0;
}