#include <bits/stdc++.h>
using namespace std;
const int N = 300004;
int n, m;
long long a[N], f[N][4];
int main() {
  scanf("%d", &n);
  long long s = 0, t = 0, z;
  for (int i = 1; i <= n; i++) {
    cin >> z;
    s += z;
    if (i & 1)
      t += z / 2;
    else
      t += z - z / 2;
  }
  printf("%lld\n", min(t, s - t));
}

### Prompt

Your challenge is to write 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 N = 300004;
int n, m;
long long a[N], f[N][4];
int main() {
  scanf("%d", &n);
  long long s = 0, t = 0, z;
  for (int i = 1; i <= n; i++) {
    cin >> z;
    s += z;
    if (i & 1)
      t += z / 2;
    else
      t += z - z / 2;
  }
  printf("%lld\n", min(t, s - t));
}

```