#include <bits/stdc++.h>
using namespace std;
const int inf = 2e9;
const int mod = 1e9 + 7;
const int N = 3e5 + 15;
int n;
long long a[2];
int main() {
  scanf("%d\n", &n);
  for (int i = 1; i <= n; ++i) {
    int x;
    scanf("%d", &x);
    a[i & 1] += x / 2 + (x & 1);
    a[i & 1 ^ 1] += x / 2;
  }
  cout << min(a[0], a[1]) << '\n';
  return 0;
}

### 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;
const int inf = 2e9;
const int mod = 1e9 + 7;
const int N = 3e5 + 15;
int n;
long long a[2];
int main() {
  scanf("%d\n", &n);
  for (int i = 1; i <= n; ++i) {
    int x;
    scanf("%d", &x);
    a[i & 1] += x / 2 + (x & 1);
    a[i & 1 ^ 1] += x / 2;
  }
  cout << min(a[0], a[1]) << '\n';
  return 0;
}

```