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

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    vector<int> a(f);
    for (int &e : a) cin >> e;
    
    int ans = k * n - accumulate(a.begin(), a.end(), 0);
    cout << max(ans, -1) << endl;
    return 0;
}