B - Simplified mahjong

Time Limit: 2 sec / Memory Limit: 256 MB

### 問題文

2 枚のカードについて、それらに書かれた整数の差の絶対値が 1 以下のとき、これらをペアにすることができます。

### 制約

• 1 ≦ N ≦ 10^5
• 0 ≦ A_i ≦ 10^9 (1 ≦ i ≦ N)
• 入力はすべて整数である。

### 入力

N
A_1
:
A_N


### 入力例 1

4
4
0
3
2


### 出力例 1

4


### 入力例 2

8
2
0
1
6
0
8
2
1


### 出力例 2

9


Score : 400 points

### Problem Statement

Snuke has a large collection of cards. Each card has an integer between 1 and N, inclusive, written on it. He has A_i cards with an integer i.

Two cards can form a pair if the absolute value of the difference of the integers written on them is at most 1.

Snuke wants to create the maximum number of pairs from his cards, on the condition that no card should be used in multiple pairs. Find the maximum number of pairs that he can create.

### Constraints

• 1 ≦ N ≦ 10^5
• 0 ≦ A_i ≦ 10^9 (1 ≦ i ≦ N)
• All input values are integers.

### Input

The input is given from Standard Input in the following format:

N
A_1
:
A_N


### Output

Print the maximum number of pairs that Snuke can create.

### Sample Input 1

4
4
0
3
2


### Sample Output 1

4


For example, Snuke can create the following four pairs: (1,1),(1,1),(3,4),(3,4).

### Sample Input 2

8
2
0
1
6
0
8
2
1


### Sample Output 2

9