#include <bits/stdc++.h>
using namespace std;
int a[300005];
void work() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
  }
  long long ans = 0;
  int tmp1 = 0;
  int tmp2 = 0;
  for (int i = 1; i <= n; i++) {
    ans += a[i] / 2;
    if (a[i] % 2 == 1) {
      if (i % 2 == 0) {
        tmp1++;
      } else {
        tmp2++;
      }
    }
  }
  printf("%lld\n", ans + min(tmp1, tmp2));
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int T;
  T = 1;
  while (T--) {
    work();
  }
}

### Prompt

Develop a solution in CPP to the problem described below:

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 a[300005];
void work() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
  }
  long long ans = 0;
  int tmp1 = 0;
  int tmp2 = 0;
  for (int i = 1; i <= n; i++) {
    ans += a[i] / 2;
    if (a[i] % 2 == 1) {
      if (i % 2 == 0) {
        tmp1++;
      } else {
        tmp2++;
      }
    }
  }
  printf("%lld\n", ans + min(tmp1, tmp2));
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int T;
  T = 1;
  while (T--) {
    work();
  }
}

```