include <bits/stdc++.h>

using namespace std;

int main() {

int n, b[40001];
cin >> n;
int a[n];
for (int i = 0; i < n; i++) {
	cin >> a[i];
}
int sum = 0;
for (int i = 0; i < n; i++) {
	for (int j = i + 1; j < n; j++) {
		b[a[i] + a[j]]++;
	}
}
for (int i = 0; i < n; i++) {
	if (b[a[i]] > 0) {
		sum++;
	}
}
cout << sum;
return 0;

}