B. Domino for Youngtime limit per test3 secondsmemory limit per test256 megabytesinputstandard inputoutputstandard outputYou are given a Young diagram. Given diagram is a histogram with n columns of lengths a_1, a_2, \ldots, a_n (a_1 \geq a_2 \geq \ldots \geq a_n \geq 1).  Young diagram for a=[3,2,2,2,1]. Your goal is to find the largest number of non-overlapping dominos that you can draw inside of this histogram, a domino is a 1 \times 2 or 2 \times 1 rectangle.InputThe first line of input contain one integer n (1 \leq n \leq 300\,000): the number of columns in the given histogram.The next line of input contains n integers a_1, a_2, \ldots, a_n (1 \leq a_i \leq 300\,000, a_i \geq a_{i+1}): the lengths of columns.OutputOutput one integer: the largest number of non-overlapping dominos that you can draw inside of the given Young diagram.ExampleInput
5
3 2 2 2 1
Output
4
NoteSome of the possible solutions for the example: 
5
3 2 2 2 1

4 
['dp', 'greedy', 'math', '*2000']