#include <cstdio>
int freq[7001],v[7001];
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n, a[9001], i;
	scanf("%d",&n);
	for(i=1;i<=n;++i)
		scanf("%d",&a[i]);

	for(i=1;i<n;++i){
		++freq[a[i]];
		for (int j = i + 1 ; j <= n ; ++j){
			if (!((a[i] + a[j])&1))
				v[((a[i] + a[j]) >> 1)]++;
		}
	}
	freq[a[n]]++;

	unsigned int total = 0;
	for(int i=1;i<=n;++i){
		if (v[a[i]])
			total += v[a[i]] - freq[a[i]] + 1;
	}

	printf("%d\n", total);

	return 0;
}