#include<iostream>
int main(void)
{
	using namespace std;
	int n;
	cin >> n;
	int a[100]{ 0 };
	int flag[10001]{0};
	for (int i = 0; i < n; i++) {
		cin >> a[i];
		flag[a[i]] = 1;
	}
	int count = 0;
	for (int i = 0; i < n - 1; i++)
		for (int j = i + 1; j < n; j++)
			if (flag[a[i] + a[j]]) {
				count++;
				flag[a[i] + a[j]] = 0;
			}
	cout << count;
	/*for(int i=0;i<n-1;i++)
		for (int j = 0; j < n - 1 - i; j++) 
			if (a[i + 1] < a[i]) {
				int p;
				p = a[i];
				a[i] = a[i + 1];
				a[i + 1] = p;
			}
	//先进行排序
	//最大的那个数不需要与任何数进行组合
	int count = 0;
	for (int i = 0; i < n-2; i++) {
		for (int j = i + 1; j < n-1; j++) {
			int p = a[i] + a[j];
			for (int k = j + 1; k < n; k++)
				if (p == a[k]) {
					count++;
					break;
				}
		}
	}*/
	return 0;
}