#include <iostream>
#include <vector>
#include <numeric>

using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    vector<int> a(f);
    for (int i = 0; i < f; i++) cin >> a[i];
    int ans = (k * n) - accumulate(a.begin(), a.end(), 0);
    cout << (ans >= 0 ? ans : -1) << endl;
}