Count how many pairs of elements have a sum equal to a given number k.

Difficulty: Hard

Phase: 4 · Basic Arrays

XP Reward: 30 XP

Topics: Basic Arrays, Hard

Supported Languages: Python, JavaScript, C, C++, Java

Problem Description

Given N followed by N integers on line 2, and integer K on line 3, count how many pairs (i < j) satisfy A[i] + A[j] = K. Input Format: Read input from standard input (stdin). Output Format: Print the result to standard output (stdout).

Constraints

-10^9 <= N <= 10^9

Examples

Example 1

Input: 5 1 5 7 -1 5 6

Output: 3

Explanation: Sample execution for input 5 1 5 7 -1 5 6