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

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

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

    if (res >= 0) {
        cout << res << endl;
    } else {
        cout << "-1" << endl;
    }
}