include <bits/stdc++.h>

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

int main() {

int n;
cin >> n;

for (int i = 1; i <= n; i++) {
	cin >> a[i];
	b	[a[i]] = 1;
}
int ans = 0;
for (int i = 1; i <= n; i++) {
	for (int j = i + 1; j <= n; j++) {
		if (b[a[i] + a[j]] == 1	) {
			ans++;
			b[a[i] + a[j]] = 0;
		}

	}
}
cout << ans;

return 0;

}