#include<iostream>

using namespace std;

int main(){
    int K, N, F, T, x;
    cin >> K >> N >> F;
    T = K * N;
    for(int i = 0; i < F; i++){
        cin >> x;
        T -= x;
    }

    if(T < 0){
        cout << -1 << endl;
    }else{
        cout << T << endl;
    }

    return 0;
}