#include<cstdio>
#include<fstream>
using namespace std;

FILE *f;
ofstream g("medie.out");

int n,a[9001],v[7001],frecv[7001],i,j,k;
long long nr;
int main()
{
	f=fopen("medie.in","r");
	fscanf(f,"%d\n",&n);
	for(i=1;i<=n;i++)
		fscanf(f,"%d\n",&a[i]);
	for(i=1;i<n;i++)
	{
		frecv[a[i]]++;
		for(j=i+1;j<=n;j++)
			if(((a[j]+a[i])&1)==0)
				v[(a[j]+a[i])>>1]++;
	}
	frecv[a[n]]++;
	for(i=1;i<=7001;i++)
		if(v[a[i]])
		{	nr+=v[a[i]]-frecv[a[i]]+1;}
	g<<nr<<'\n';
	fclose(f);
	g.close();
	return 0;
}