This Message Will Self-Destruct in 5s

AtCoder

IDabc166_e

Time2000ms

Memory256MB

Difficulty—

You are the top spy of AtCoder Kingdom. To prevent the stolen secret from being handed to AlDebaran Kingdom, you have sneaked into the party where the transaction happens. There are $N$ attendees in the party, and they are given attendee numbers from $1$ through $N$. The height of Attendee $i$ is $A_i$. According to an examination beforehand, you know that a pair of attendees satisfying the condition below will make the transaction. * The absolute difference of their attendee numbers is equal to the sum of their heights. There are $\frac{N(N-1)}{2}$ ways to choose two from the $N$ attendees and make a pair. Among them, how many satisfy the condition above? P.S.: We cannot let you know the secret. ## Constraints * All values in input are integers. * $2 \leq N \leq 2 \times 10^5$ * $1 \leq A_i \leq 10^9\ (1 \leq i \leq N)$ ## Input Input is given from Standard Input in the following format: $N$ $A_1$ $A_2$ $\dots$ $A_N$ [samples]

Samples

Input #1

6
2 3 3 1 3 1

Output #1

3

*   $A_1 + A_4 = 3$, so the pair of Attendee $1$ and $4$ satisfy the condition.
*   $A_2 + A_6 = 4$, so the pair of Attendee $2$ and $6$ satisfy the condition.
*   $A_4 + A_6 = 2$, so the pair of Attendee $4$ and $6$ satisfy the condition.

No other pair satisfies the condition, so you should print $3$.

Input #2

6
5 2 4 2 8 8

Output #2

0

No pair satisfies the condition, so you should print $0$.

Input #3

32
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5

Output #3

API Response (JSON)

{
  "problem": {
    "name": "This Message Will Self-Destruct in 5s",
    "description": {
      "content": "You are the top spy of AtCoder Kingdom. To prevent the stolen secret from being handed to AlDebaran Kingdom, you have sneaked into the party where the transaction happens. There are $N$ attendees in t",
      "description_type": "Markdown"
    },
    "platform": "AtCoder",
    "limit": {
      "time_limit": 2000,
      "memory_limit": 262144
    },
    "difficulty": "None",
    "is_remote": true,
    "is_sync": true,
    "sync_url": null,
    "sign": "abc166_e"
  },
  "statements": [
    {
      "statement_type": "Markdown",
      "content": "You are the top spy of AtCoder Kingdom. To prevent the stolen secret from being handed to AlDebaran Kingdom, you have sneaked into the party where the transaction happens.\nThere are $N$ attendees in t...",
      "is_translate": false,
      "language": "English"
    }
  ]
}

Full JSON Raw Segments