import java.util.*;
class Main{
 
    public static int pairs(int [] arr,int k){
        HashMap<Integer,Integer> map=new HashMap<>();
        int count=0;
        for(int i=0;i<arr.length;i++){
            int diff=k-arr[i];
            count+=map.getOrDefault(diff,0);
            map.put(arr[i],map.getOrDefault(arr[i],0)+1);
        }
        return count;
    }
    public static void main (String[] args) {
        int [] arr={1,2,3,4,1,5};
        int k=5;
       System.out.println(pairs(arr,k));
    }
}

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