#include <bits/stdc++.h>
using namespace std;
double PI = 3.141592653589793238462643383279;
const double EPS = 1e-9;
const long long MOD = 1000000007;
const int inf = 1 << 30;
const long long linf = 1LL << 60;
int n;
long long g, k;
int main() {
  scanf("%d", &n);
  for (int i = 0; i < (int)n; i++) {
    long long a;
    scanf("%lld", &a);
    if (i % 2) {
      g += (a + 1) / 2;
      k += a / 2;
    } else {
      g += a / 2;
      k += (a + 1) / 2;
    }
  }
  printf("%lld\n", min(g, k));
}

### Prompt

In CPP, your task is to solve 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;
double PI = 3.141592653589793238462643383279;
const double EPS = 1e-9;
const long long MOD = 1000000007;
const int inf = 1 << 30;
const long long linf = 1LL << 60;
int n;
long long g, k;
int main() {
  scanf("%d", &n);
  for (int i = 0; i < (int)n; i++) {
    long long a;
    scanf("%lld", &a);
    if (i % 2) {
      g += (a + 1) / 2;
      k += a / 2;
    } else {
      g += a / 2;
      k += (a + 1) / 2;
    }
  }
  printf("%lld\n", min(g, k));
}

```