#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

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

    int beans_count = K * N;

    int A;

    for (int i = 0; i < F; i++) {
        cin >> A;
        beans_count -= A;
    }

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

    return 0;
}