#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int c;
  cin >> c;
  long long int black = 0, white = 0;
  for (long long int(i) = (0); (i) < (c); (i)++) {
    long long int row;
    cin >> row;
    if (i % 2 == 0) {
      long long int w = row / 2;
      long long int b = row - w;
      black += b;
      white += w;
    } else {
      long long int b = row / 2;
      long long int w = row - b;
      black += b;
      white += w;
    }
  }
  cout << min(white, black);
  return 0;
}

### Prompt

In CPP, your task is to solve 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() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int c;
  cin >> c;
  long long int black = 0, white = 0;
  for (long long int(i) = (0); (i) < (c); (i)++) {
    long long int row;
    cin >> row;
    if (i % 2 == 0) {
      long long int w = row / 2;
      long long int b = row - w;
      black += b;
      white += w;
    } else {
      long long int b = row / 2;
      long long int w = row - b;
      black += b;
      white += w;
    }
  }
  cout << min(white, black);
  return 0;
}

```