#include <bits/stdc++.h>

using namespace std;

int main() {
    int K, N, F;
    cin >> K >> N >> F;

    int x = K * N;
    for (int i = 0; i < F; ++i) {
        int a;
        cin >> a;
        x -= a;
    }

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