#include <iostream>
#include <string>
using namespace std;

int
main() {
        int k, n, f;
        cin >> k >>  n >> f;
        int a[f];
        for (int i = 0; i < f; i++) cin >> a[i];
        int b = k * n;
        for (int i = 0; b > 0 && i < f; i++) {
                b -= a[i];
        }
        if (b >= 0) cout << b << endl;
        else cout << -1 << endl;
        return 0;
}