#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  long long a[n];
  long long white = 0, black = 0, white_black = 0;
  for (long long col = 0; col < n; col++) {
    long long x;
    cin >> x;
    white_black += x / 2;
    if (x % 2 == 1) {
      if (col % 2 == 0) {
        white++;
      } else {
        black++;
      }
    }
  }
  cout << white_black + min(white, black);
}

### Prompt

Generate a Cpp solution to the following problem:

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>

### Response

```cpp
#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  long long a[n];
  long long white = 0, black = 0, white_black = 0;
  for (long long col = 0; col < n; col++) {
    long long x;
    cin >> x;
    white_black += x / 2;
    if (x % 2 == 1) {
      if (col % 2 == 0) {
        white++;
      } else {
        black++;
      }
    }
  }
  cout << white_black + min(white, black);
}

```