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

int main()
{
    int K, N, F;
    int age_sum = 0;
    cin >> K >> N >> F;
    for (int i =0; i < F; ++i){
        int a;
        cin >> a;
        age_sum += a; 
    }

    if (K * N - age_sum >= 0){
        cout << K * N - age_sum << endl;
    }else{
        cout << "-1" << endl;
    }

    return 0; 
}