#include <bits/stdc++.h>
using namespace std;

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