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

class Codechef
{
	public static void main (String[] args) throws java.lang.Exception
	{
	    /*
	        number of unordered quadruplets (i,j,k,l) such that b[i] + b[j] + b[k] + b[l] = 0
	    
	    */
	    
	    
	    //O(n^4) ;
	    Scanner sc = new Scanner(System.in) ; 
	    int n = sc.nextInt() ; 
	    int arr[] = new int[n] ; 
	    
	    for(int i = 0 ; i < n ; i++) {
	        arr[i] = sc.nextInt() ; 
	    }
	    
	    
	    Map<Integer , Integer > map = new HashMap<>() ; 
	    
	    for(int i = 2 ; i < n ; i++) {
	        for(int j = i + 1 ; j < n ; j++) {
	            map.put(arr[i] + arr[j] , map.getOrDefault(arr[i] + arr[j] , 0) + 1) ; 
	        }
	    }
	    int count = 0 ;
	   
	   
	   for(int i = 1 ; i < n - 1; i++) {
	       for(int j = 0 ; j < i ; j++  ) {
	           int sum = 0 - (arr[i] + arr[j] ); 
	           if(map.containsKey(sum)) {
	               count += map.get(sum) ; 
	           }
	       }
	       
	       for(int k = i + 2 ; k < n ; k++) { //remove pair for that element wich is going to become i next 
	           map.put(arr[i+1] + arr[k] , map.get(arr[i+1] + arr[k] ) - 1 ) ; 
	       }
	   }
	    
	    System.out.println(count);
	}
}