#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
int ans[20005] = { 0 };
bool in[10005] = {0};
int main()
{
	int n,arr[105],cnt = 0,maxx = 0;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> arr[i];
		in[arr[i]] = 1;
	}
	for (int i = 0; i < n; i++)
	{
		for (int j = i + 1; j < n; j++)
		{
			ans[arr[i] + arr[j]]++;
			maxx = max(maxx, arr[i] + arr[j]);
		}
	}
	for (int i = 1; i <= maxx; i++)
	{
		if (in[i] && ans[i] != 0)
		{
			cnt++;
		}
	}
	cout << cnt << endl;
	system("pause");
	return 0;
}