#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:64777216")
using namespace std;
const int inf = (int)1e9 + 5;
const long long INF = (long long)1e18 + 5;
void fail() {
  cout << -1 << endl;
  exit(0);
}
void solve() {
  long long n, U;
  cin >> n >> U;
  vector<long long> E(n);
  for (long long i = 0; i < n; i++) cin >> E[i];
  bool has_ans = false;
  pair<long long, long long> best;
  for (long long i = 0; i + 2 < n; i++) {
    long long j = i + 1;
    auto it = upper_bound(E.begin(), E.end(), E[i] + U);
    if (it == E.begin()) continue;
    --it;
    long long ek = *it;
    if (ek <= E[j]) continue;
    pair<long long, long long> cur = {E[j] - E[i], ek - E[i]};
    if (!has_ans) {
      has_ans = true;
      best = cur;
    }
    if (cur.first * best.second < cur.second * best.first) best = cur;
  }
  if (!has_ans) fail();
  double ans = 1 - (double)best.first / (double)best.second;
  cout << ans;
}
int main() {
  string filename = "acm";
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(15);
  solve();
  (void)0;
  return 0;
}
