#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
    int n,d,t;
    cin >> n >> d;
    for(int i = 0 ; i < n; ++ i){
        cin >> t;
        d-=t;
    }
    d-=(n-1)*10;
    if(d < 0) cout<<-1<<endl;
    else cout<<(n-1)*2+d/5<<endl;
}