#include <bits/stdc++.h>
using namespace std;
bool prime(long long n) {
  for (int i = 2; i <= sqrt(n); i++)
    if (n % i == 0) return false;
  return true;
}
int main() {
  string s, s1;
  long long a, b, c, m, n, k, t, l, d, p, nl, np;
  t = 1;
  while (t--) {
    cin >> n >> d;
    vector<long long> arr(n);
    long long sum = 0;
    for (int i = 0; i < n; i++) {
      cin >> arr[i];
      sum += arr[i];
    }
    if (sum + (n - 1) * 10 > d)
      cout << -1 << endl;
    else {
      cout << (n - 1) * 2 + (d - (sum + (n - 1) * 10)) / 5 << endl;
    }
  }
}
