Background

From Northwestern Europe Regional Contest (NWERC) 2017 Problem K.

Original problem license: CC BY-SA.

Problem Description

Laura is organising a knockout tournament, in which her friend Dale takes part. Laura would like to maximise the probability of Dale winning the tournament by arranging the games in a favourable way. She does not know how to do it, so she asked you for help. Naturally, you refuse to cooperate with such a deplorable act---but then you realise that it is a very nice puzzle!

When the number of players is a power of two, the tournament setup can be described recursively as follows: the players are divided into two equal groups that each play their own knockout tournament, after which the winners of both tournaments play each other. Once a player loses, they are out of the tournament.

When the number of players is not a power of two, some of the last players in the starting line-up advance from the first round automatically so that in the second round the number of players left is a power of two, as shown in Figure 1.

Figure 1: A tournament tree with $5$ players. Players C, D, and E advance from the first round automatically.

Every player has a rating indicating their strength. A player with rating $a$ wins a game against a player with rating $b$ with probability $\frac{a}{a+b}$ (independently of any previous matches played).

Laura as the organiser can order the starting line-up of players in any way she likes. What is the maximum probability of Dale winning the tournament?

Input Format

The input consists of one line with an integer $n$ ($2 \le n \le 4096$), the total number of players, followed by $n$ lines, each with an integer $r$ ($1 \le r \le 10^5$), the rating of a player. The first rating given is Dale's rating.

Output Format

Output the maximum probability with which Dale can win the tournament given a favourable setup. Your answer should have an absolute or relative error of at most $10^{-6}$.

4
3
1
2
4

0.364285714

5
1
1
3
3
3

0.125