#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

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