#include <iostream>

using namespace std;

int main() {
    int k, n, f, a, sum = 0, leftovers;
    cin >> k >> n >> f;

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

    leftovers = k*n - sum;

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

    return 0;
}