#include <iostream>
using namespace std;

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