#include <iostream>
using namespace std;
int main() {
    int k, n, f; cin >> k >> n >> f;
    int a = 0;
    for (int i = 0; i < f; ++i) {
        int c; cin >> c;
        a += c;
    }
    if (a > k * n) {
        cout << -1 << endl;
    }
    else {
        cout << k * n - a << endl;
    }
}