#include <iostream>
#include <vector>

using namespace std;

int main(int argc, char const* argv[]) {
    int k, n, f, total;

    cin >> k >> n >> f;
    total = k * n;

    int a;
    for (int i = 0; i < f; i++) {
        cin >> a;
        total -= a;
    }

    if (total < 0) total = -1;
    cout << total << endl;
    return 0;
}