#include <iostream>
using namespace std;

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