//Given an array arr[N],, check if any pair (i,j) exist such that arr[i] + arr[j]==K , and i!=j;Count these kind of pair 

import java.util.*;
import java.lang.*;
import java.io.*;

// The main method must be in a class named "Main".
class Main {

    public static int countPairSum(int [] arr , int k ){
        
        int count = 0 ;
        HashMap<Integer,Integer> fmap = new HashMap<>();
        for(int i=0; i < arr.length ; i++){
            int fval = arr[i];
            int sval = k - arr[i];

            //Check the frq of sval in past 
            if(fmap.containsKey(sval) ){
                count+= fmap.get(sval);
            }

            //increase the freq of current element 
            if(fmap.containsKey(fval)){
                int oldFreq = fmap.get(fval);
                fmap.put(fval,oldFreq+1);
            }
            else{
                fmap.put(fval,1);
            }            
        }
        return count;      
    }
    
    public static void main(String[] args) {
        int arr[] = {2, 5 , 2 , 5 , 8 , 5 , 2 , 8 };
        int k = 10;
        
        System.out.println("Final Count " + countPairSum(arr,k));
    }
}