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

int main() {
    int k, n ,f, r;
    cin >> k >> n >> f;
    vector<int> a(f);
    for (int i=0; i<f; i++) cin >> a[i];
    int max = k * n;
    r = k * n - accumulate(a.begin(), a.end(), 0);
    r = r>=0 ? r : -1;
    cout << r << endl;
}