#include<vector>
#include <iostream>
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 sum = k * n;
    for (int i = 0; i < f; ++i) {
        sum-= a[i];
        if (sum < 0) {
            cout << "-1" << endl;
            return 0;
        }
    }
    cout << sum << endl;
}