#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
using namespace std;

int main(void) {
  int i, j, k, l, n;
  double ans = 0;
  scanf("%d", &n);
  int a[n];
  pair<int, int> d[n];
  for(i = 0; i < n; ++i) scanf("%d", &a[i]);
  sort(a, a + n, greater<int>());
  for(i = 0; i < n - 1; ++i) d[i].first = a[i] - a[i + 1], d[i].second = i;
  sort(d, d + n - 1);
  if(d[0].second && d[0].second != 1) ans = (double)(a[0] + a[1]) / d[0].first;
  else {
    for(i = 0; i < 4; ++i) for(j = 0; j < 4; ++j) for(k = 0; k < 4; ++k) for(l = 0; l < 4; ++l) if(i != j && j != k && k != l && l != i && i != k && j != l) if(ans < (double)(a[i] + a[j]) / (a[k] - a[l])) ans = (double)(a[i] + a[j]) / (a[k] - a[l]);
    if(d[1].second && d[1].second != 1) {
      if(ans < (double)(a[0] + a[1]) / d[1].first) ans = (double)(a[0] + a[1]) / d[1].first;
    } else {
      if(ans < (double)(a[0] + a[1]) / d[2].first) ans = (double)(a[0] + a[1]) / d[2].first;
    }
  }
  printf("%f\n", ans);
  return 0;
}
