#include <iostream>
using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int sum = 0;
    for(int i=0;i<f;i++) {
        int ai;
        cin >> ai;
        sum += ai;
    }
    if(k * n < sum)
        cout << -1 << endl;
    else
        cout << k *n - sum << endl;
}