#include <iostream>

#include <vector>

#include <algorithm>

using namespace std;

int main(){

    int n,d,t;

    cin >> n >> d;

    for(int i =  ; i < n; ++ i){

        cin >> t;

        d-=t;

    }

    d-=(n-)*;

    if(d < ) cout<<-<<endl;

    else cout<<(n-)*+d/<<endl;

}