题目描述

You are given an integer sequence $A=(A _ 1,A _ 2,\ldots,A _ N)$ of length $N$.

Find how many pairs of integers $(i,j)\ (1\le i\lt j\le N)$ satisfy $j-i=A _ i+A _ j$.

输入格式

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

$N$

$A_1$ $A_2$ $\ldots$ $A_N$

输出格式

Output the answer.

样例

9
3 1 4 1 5 9 2 6 5

3

For example, when $(i,j)=(4,7)$, we have $j-i=7-4=3$ and $A _ i+A _ j=1+2=3$, so $j-i=A _ i+A _ j$.

In contrast, when $(i,j)=(3,8)$, we have $j-i=8-3=5$ and $A _ i+A _ j=4+6=10$, so $j-i\ne A _ i+A _ j$.

Only the three pairs $(i,j)=(1,9),(2,4),(4,7)$ satisfy the condition, so output 3.

3
123456 123456 123456

0

30
8 3 6 4 9 6 5 6 5 6 3 4 7 3 7 4 9 8 5 8 3 6 8 8 4 5 5 5 6 5

17

数据范围与提示

• $1\le N\le2\times10 ^ 5$
• $1\le A _ i\le2\times10 ^ 5\ (1\le i\le N)$
• All input values are integers.