#include <iostream>
using namespace std;

int main(void) {
    int k, n, f;
    cin >> k >> n >> f;
    int a;
    int beans = k * n;
    for (int i = 0; i < f; i++) {
        cin >> a;
        beans -= a;
    }
    beans = (beans < 0) ? -1 : beans;
    cout << beans << endl;
    return 0;
}