#include <iostream>
using namespace std;

int main(){
    int k, n, f;
    cin >> k >> n >> f;
    int s = 0;
    int temp;
    for(int i = 0; i < f; i++){
        cin >> temp;
        s += temp;
    }
    if(n*k < s) cout << "-1" << endl;
    else cout << n*k - s << endl;

    return 0;
}