#include <iostream>
#include <vector>

using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int mameall = k * n;
    
    vector<int> v(f);
    int agesum = 0;
    for (int i = 0; i < f; ++i) {
        cin >> v.at(i);
        agesum += v.at(i);
    }
    if (agesum > mameall) {
        cout << -1 << endl;
    } else {
        cout << mameall - agesum << endl;
    }
    return 0;
}