#include <iostream>
using namespace std;

int main() {
    int k, n, f;    cin >> k >> n >> f;
    int beansNum = k * n;
    int family;

    for (int i = 0; i < f; ++i) {
        cin >> family;
        beansNum -= family;
        if(beansNum < 0){
            cout << -1 << endl;
            return 0;
        }
    }

    cout << beansNum << endl;
    return 0;
}