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

int main()
{
    int K, N, F;
    cin >> K >> N >> F;
    int count = K * N;
    for (int i = 0; i < F; i++)
    {
        int A;
        cin >> A;
        count -= A;
    }
    if (count < 0)
    {
        cout << -1;
    }
    else
    {
        cout << count;
    }
}