#include<bits/stdc++.h>
using namespace std;

int triplets(vector<int>arr) {
	map<int, int> k;
	map<pair<int, int>, int> mpp;
	vector<vector<int >> ssd;
	int count = 0;
	sort(arr.begin(), arr.end());
	int i = 0;
	while (i < arr.size()) {
		if (k.find(arr[i]) == k.end()) {
			k[arr[i]] = 1;
		}
		i += 1;
	}
	int j = 0;
	while (j < arr.size() - 1) {
		int q = j + 1;
		while (q < arr.size()) {
			if (k.find(arr[j] + arr[q]) != k.end() and mpp[ {arr[j], arr[q]}] != arr[j] + arr[q]) {
				count += 1;
				ssd.push_back({arr[j], arr[q], arr[j] + arr[q]});
				mpp[ {arr[j], arr[q]}] = arr[j] + arr[q];
			}
			q += 1;
		}
		j += 1;
	}
	return count;
}

int main() {

	int n;
	cin >> n;

	vector<int> arr(n);
	for (int i = 0; i < n; i++) {
		cin >> arr[i];
	}
	int count = triplets(arr);
	cout << count << endl;
}