#include <iostream>

using namespace std;

int main() {
    int K, N, F, a, A, result;
    cin >> K >> N >> F;
    for(int i = 0; i < F; i++){
        cin >> a;
        A += a;    
    }
    result = K * N - A;
    if(result < 0){
        cout << -1 << endl;
    }else{
        cout << result << endl;
    }
    return 0;
}