#include <stdio.h>
#include <vector>

using namespace std;

int n;
int result,x,i;
vector<int> a;
vector<int> cnt;


int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
    scanf("%d", &n);

	a.push_back(0);
	
	for (i=0;i<n;i++)
	{
		scanf("%d", &x);
		a.push_back(x);
	}
	
	a.resize(16000);
	cnt.resize(16000);
	
	for (int i=1;i<=n;i++)
     for (int j=i+1;j<=n;j++)
     {
			int middle = a[i]+a[j];
				cnt[middle]++;
			if (middle == a[i]*2)
				result--;  
     }
     
     for (int i=1;i<=n;++i)
	 {
		  result +=	cnt[a[i]*2];
          cnt[a[i]*2]--;
	 }
     printf("%d", result);
     
     return 0;     
}