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

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

int n,a[10001],frecv[10001],i,j;
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]);frecv[a[i]]++;
	}
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
		{
			int x=a[j]+a[i];
			if((x&1)==0)
			{
				x=(a[j]+a[i])>>1;
				nr+=frecv[x];
				if(a[i]==x)
					nr-=2;
			}
		}
	g<<nr<<'\n';
	fclose(f);
	g.close();
	return 0;
}