#include <iostream>
using namespace std;

int main()
{
    int K, N, F;
    cin >> K >> N >> F;
    int ans = K * N;
    for (int i = 0; i < F; i++)
    {
        int tmp;
        cin >> tmp;
        ans -= tmp;
        if (ans < 0)
        {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << ans << endl;
    return 0;
}