var input = require('fs').readFileSync('/dev/stdin', 'utf8');
var arr=input.trim().split("\n");
arr.shift();
var a = arr.shift().split(" ").map(Number);
a.sort(function(a,b){
   return b-a;
});
var s = [];
a.reduce(function(a,b){
   s.push([a-b,a,b]); 
   return b;
});
var max = 0;
a.reduce(function(a,b){
   s.forEach(function(v){
      if(v[1] != a && v[2] != b && v[1] != b && v[2] != a){
         max = Math.max(max, (a+b)/v[0]).toFixed(6);
      }
   });
   return b;
});
console.log(max);