#include <bits/stdc++.h>
using namespace std;
template <class T1, class T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "{" << p.first << "," << p.second << "}";
  return os;
}
const int N = 3e5 + 5;
const int oo = 1e9 + 7;
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  long long n, y, z, ans = 0;
  y = z = 0;
  cin >> n;
  while (n--) {
    long long x;
    cin >> x;
    ans += x / 2;
    if (n % 2)
      y += x % 2;
    else
      z += x % 2;
  }
  ans += min(y, z);
  cout << ans;
}

### 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;
template <class T1, class T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "{" << p.first << "," << p.second << "}";
  return os;
}
const int N = 3e5 + 5;
const int oo = 1e9 + 7;
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  long long n, y, z, ans = 0;
  y = z = 0;
  cin >> n;
  while (n--) {
    long long x;
    cin >> x;
    ans += x / 2;
    if (n % 2)
      y += x % 2;
    else
      z += x % 2;
  }
  ans += min(y, z);
  cout << ans;
}

```