/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		int[] b = {1, 2, 3, 4, 5};
        int k = 6;
        int count = 0;
        HashMap<Integer,Integer> mp = new HashMap<>();
        for(int i = 0; i < b.length; i++){
        	int diff = k- b[i]; 
        	if(mp.containsKey(diff)){
        		count++;
        	}
        	mp.put(b[i],i);
        }
        System.out.print(count);
	}
}

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