#include <algorithm>
#include <numeric>
#include <string>
#include <vector>
#include <iostream>
#include <queue>
#include <utility>
#include <cmath>
using namespace std;

using ull = unsigned long long;

int main()
{
    int k,n,f;
    cin >> k >> n >> f;

    vector<int> family(f);

    int sum = k*n;

    for(auto&& x : family)
    {
        cin >> x;
        sum -= x;
    }

    if(sum < 0)
        cout << -1 << endl;
    else
        cout << sum << endl;
}