#include <bits/stdc++.h>
using namespace std;
long long int N = 1e9 + 7;
void solve() {
  long long int n;
  cin >> n;
  vector<long long int> a(n + 4);
  long long int tot = 0;
  for (long long int i = 1; i <= n; i++) {
    cin >> a[i];
    tot += a[i];
  }
  long long int cnt = 0;
  for (long long int i = 1; i <= n; i++) {
    if (i % 2 == 0) {
      if (a[i] % 2) {
        cnt++;
      }
    } else {
      if (a[i] % 2) {
        cnt--;
      }
    }
  }
  cout << (tot - abs(cnt)) / 2 << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int t = 1;
  while (t--) {
    solve();
  }
}

### Prompt

Please formulate 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;
long long int N = 1e9 + 7;
void solve() {
  long long int n;
  cin >> n;
  vector<long long int> a(n + 4);
  long long int tot = 0;
  for (long long int i = 1; i <= n; i++) {
    cin >> a[i];
    tot += a[i];
  }
  long long int cnt = 0;
  for (long long int i = 1; i <= n; i++) {
    if (i % 2 == 0) {
      if (a[i] % 2) {
        cnt++;
      }
    } else {
      if (a[i] % 2) {
        cnt--;
      }
    }
  }
  cout << (tot - abs(cnt)) / 2 << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int t = 1;
  while (t--) {
    solve();
  }
}

```