#include <bits/stdc++.h>
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 sum_bean = 0;

    for (int i = 0; i < F; i++) {
        sum_bean += A[i];
    }

    cout << max(-1, K * N - sum_bean) << endl;

    return 0;
}