#include <iostream>

using namespace std;


int main() {
    int k, n, f;
    int input[100];
    int ans = 0;
    int sum = 0;

    cin >> k >> n >> f;
    for (int i = 0; i < f; i++){
        cin >> input[i];
        sum += input[i];
    }
    if (k * n >= sum){
        cout << k * n - sum << "\n";
    }
    else {
        cout << -1 << "\n";
    }
    
    return 0;
}