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

int main() {
    int K, N, F, sum = 0;
    cin >> K >> N >> F;

    vector<int> A(F);
    for(int i = 0;i< F; i++){
        cin >> A.at(i);
        sum += A.at(i);
    }

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