#include <iostream>
using namespace std;

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