#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a;
  cin >> n >> a;
  int arr[n];
  int sum = 0;
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
    sum += arr[i];
  }
  sum += (n - 1) * 10;
  if (sum > a) {
    cout << -1 << endl;
  } else {
    cout << (n - 1) * 2 + (a - sum) / 5 << endl;
  }
  return 0;
}
