include

define M 105

define m 20005

using namespace std; int a[M], b[m];

int main() {

int n, t, c = 0;
cin >> n;
for (int i = 0; i < 20005; i++)
	b[i] = 0;

for (int i = 1; i <= n; i++)
	scanf("%d", &a[i]);
for (int i = 1; i <= n; i++) {
	for (int j = i + 1; j <= n; j++) {
		t = a[i] + a[j];
		b[t]++;
	}
}
for (int i = 0; i < 20005; i++) {
	if (b[i] > 0) {
		for (int j = 1; j <= n; j++) {
			if (i == a[j]) {
				c++;
				break;
			}
		}
	}
}
printf("%d", c);
return 0;

}