Dr.Sankey wants to form 2-member teams for future programming contests in his university which has $N$ students. Each student has a skill value and the total power of a team is half of the sum of skill values of the team members. He forms teams in such a manner: The 1^st and 2^nd highest skilled students form a team together, the 3^rd and 4^th highest skilled form a team together and so on. Each student would be a part of some team i.e; $N$/2 teams are made.

Find the average power of the teams in Dr.Sankey's university.

Input Format:

First line contains the integer $N$. Next line contains $N$ integers, each denoting the skill value of i^th student.

Output Format:

Output one integer which is a floored value of the floating number denoting the average power of teams in the college.

Constraints:

1 <= $N$ <= 10⁶

1 <= A_i <= 10¹⁷

$N$ is always even 

All Necessary Calculation to find out the required answer can be done using a 64-bit integer.