#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

int main()
{
    int k, n, f, a;
    cin >> k >> n >> f;

    int age=0;
    for(int i=0; i<f; i++){
        cin >> a;
        age += a;
    }

    cout << (k*n < age ? -1 : k*n - age) << endl;

    return 0;
}