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 sys
from collections import Counter

# inf = open('input.txt', 'r')
# reader = (map(int, line.split()) for line in inf)
reader = (map(int, line.split()) for line in sys.stdin)
input = reader.__next__


n, = input()
a = list(input())

def nDominos(a, n):
    w = 0
    b = 0
    for i, el in enumerate(a):
        w += el // 2
        b += el // 2
        if el % 2 == 1:
            if i % 2 == 1:
                w += 1
            else:
                b += 1
    return min(w, b)

ans = nDominos(a, n)
print(ans)

# inf.close()
