#include <bits/stdc++.h>
using namespace std;

int main()
{
    cin.tie(0)->sync_with_stdio(0);

    int K, N, F;
    cin >> K >> N >> F;

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

    int beans = K * N;
    if (beans < A) {
        cout << -1 << endl;
    } else {
        cout << beans - A << endl;
    }

    return 0;
}