Contrary to common belief, Koalas are very socially active. Ken Koala has been on many matchmaking sites, and is now interested in making his own matchmaking algorithm. Ken's algorithm takes in a koala's favourite number, , and chooses an optimal integer such that the average of and , denoted by , is an integer and is maximal. The Koala with favourite number will be matched with a Koala with favourite number .
Given the inputted value of , you are to find the value of the integer, .
The input will contain an integer .
Output the value of the integer .
Explanation for Sample
The optimal to choose is , resulting in an average of .