#include <iostream>
#include <fstream>
#define MIN 10000
using namespace std;

int hash2[MIN + 1] = { 0 };
int count = 0;

void readData()
{
	ifstream fin("2sum.txt");
	int temp = 0;
	while(fin>>temp)
	{
		if(temp < MIN)
			hash2[temp]++;
	}
}

bool hashMap(int n)
{
	if(n > MIN)
		return false;
	if(hash2[n])
		return true;
	else
		return false;
}

int main()
{
	readData();

	for(int i = -10000; i <= 10000; i++)
	{
		for(int j = -10000; j <= 10000; j++)
		{
			if(hashMap(j) && hashMap(i - j))
			{
				count++;
				break;
			}
		}
	}
	cout<<count<<endl;
	return 0;
}