Denis has to prepare the Ural State University 90th anniversary firework.
He bought n rockets and started to think of the way he should
launch them. After a pair of sleepless nights he invented the following
algorithm.
All n rockets are placed on the surface in a single line. The interval
between two consecutive salvos is ten seconds. The leftmost and the rightmost
rocket are launched in the first salvo. After i salvos are fired, all
nonempty segments between two neighboring launched rockets are considered.
One rocket is chosen randomly and uniformly at each of these segments.
All chosen rockets are launched in the (i + 1)st salvo. Algorithm runs
until all rockets are launched.
Calculate the average duration in seconds of such a firework.
Input
The only input line contains an integer n (3 ≤ n ≤
400), which is the number of rockets bought by Denis.
Output
Output the expected duration of the firework in seconds, with
absolute or relative error not exceeding 10^{−6}.
Sample
input  output 

5
 26.66666666666

Notes
First, the rockets with numbers 1 and 5 are launched. 10 seconds later
the rocket 3 is launched with probability 1/3; in that case, 10 more
seconds later the rockets 2 and 4 are launched, and the firework is over after 20
seconds. In case the rocket 2 or rocket 4 is launched in the second salvo
(this happens with probability 2/3), the firework is over after 30 seconds.
Problem Author: Alex Samsonov
Problem Source: XV Open USU Championship