#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

int main()
{
    int K, N, F, A, sum_of_ages;
    int sum_of_beens = 0;
    vector<int> ages;
    cin >> K >> N >> F;
    for (int i=0; i<F; ++i) {
        cin >> A;
        ages.push_back(A);
    }
    sum_of_beens = K*N;
    for (int i=0; i<ages.size(); ++i) {
        sum_of_beens -= ages[i];
    }
    if (sum_of_beens < 0) cout << -1 << endl;
    else cout << sum_of_beens << endl;
        
    return 0;
}