#include <bits/stdc++.h>
using namespace std;
long long int mod = 998244353;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n, f, s = 0, t = 0, p = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> f;
    t += f;
    s ^= 1;
    if (f % 2 == 0)
      p += f / 2;
    else {
      p += f / 2;
      if (f % 2 == 1 && s == 1) p += 1;
    }
  }
  cout << min(p, t - p) << endl;
  return (0);
}

### Prompt

Construct a cpp code solution to the problem outlined:

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;
long long int mod = 998244353;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n, f, s = 0, t = 0, p = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> f;
    t += f;
    s ^= 1;
    if (f % 2 == 0)
      p += f / 2;
    else {
      p += f / 2;
      if (f % 2 == 1 && s == 1) p += 1;
    }
  }
  cout << min(p, t - p) << endl;
  return (0);
}

```