#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
using namespace std;

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