include

include

using namespace std; int a[105]; int b[20005];

int main() {

int n;
cin >> n;
memset(b, 0, sizeof(b));
for (int i = 1; i <= n; i++) {
	cin >> a[i];
}
for (int i = 1; i <= n; i++) {
	for (int j = i + 1; j <= n; j++) {
		int t = a[i] + a[j];
		b[t]++;
	}
}
int ans = 0;
for (int i = 0; i <= 20000; i++) {
	if (b[i] != 0) {
		for (int j = 1; j <= n; j++) {
			if (i == a[j]) {
				ans++;
				break;
			}
		}
	}
}
cout << ans;
return 0;

}