You are given a Young diagram. 

Given diagram is a histogram with n columns of lengths a_1, a_2, …, a_n (a_1 ≥ a_2 ≥ … ≥ a_n ≥ 1).

<image> 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 × 2 or 2 × 1 rectangle.

Input

The first line of input contain one integer n (1 ≤ n ≤ 300 000): the number of columns in the given histogram.

The next line of input contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ 300 000, a_i ≥ a_{i+1}): the lengths of columns.

Output

Output one integer: the largest number of non-overlapping dominos that you can draw inside of the given Young diagram.

Example

Input


5
3 2 2 2 1


Output


4

Note

Some of the possible solutions for the example:

<image> <image>
import os, sys, math
import collections

if os.path.exists('testing'):
	name = os.path.basename(__file__)
	if name.endswith('.py'):
		name = name[:-3]
	src = open(name + '.txt', encoding='utf8')
	input = src.readline

def solve():
	counts = []
	for o in (0, 1):
		cc = 0
		for x in data:
			cc += (x + o) // 2
			o = 1 - o
		counts.append(cc)
	return min(counts)

def integers():
	return map(int, input().strip().split()) 

def array_of_integers(q):
	return [
		tuple(map(int, input().strip().split())) for _ in range(q)
	]

n = integers()
data = list(integers())

res = solve()
print(res)
