#include <bits/stdc++.h>
#pragma GCC optimize(3, "Ofast", "inline")
using namespace std;
const long long maxn = (long long)2e5 + 5;
const long long mod = 998244353;
const long long inf = 0x3f3f3f3f;
long long T = 1;
void solve() {
  long long n;
  cin >> n;
  long long sum = 0, num = 0;
  for (long long i = 1; i <= n; ++i) {
    long long x;
    cin >> x;
    sum += x;
    num += x / 2 + ((x & 1) && (i & 1));
  }
  cout << min(num, sum - num);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  while (T--) solve();
  return 0;
}

### Prompt

Create a solution in Cpp for 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>
#pragma GCC optimize(3, "Ofast", "inline")
using namespace std;
const long long maxn = (long long)2e5 + 5;
const long long mod = 998244353;
const long long inf = 0x3f3f3f3f;
long long T = 1;
void solve() {
  long long n;
  cin >> n;
  long long sum = 0, num = 0;
  for (long long i = 1; i <= n; ++i) {
    long long x;
    cin >> x;
    sum += x;
    num += x / 2 + ((x & 1) && (i & 1));
  }
  cout << min(num, sum - num);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  while (T--) solve();
  return 0;
}

```