#include<iostream.h>
#include<fstream.h>

ifstream f("medie.in");
ofstream g("medie.out");

int main ()
{
	int x,i,k,n,V[7001],c=0,max,min;
	for(i=1;i<=7000;i++)V[i]=0;
	f>>n;
	max=7000;
	min=1;
	for(i=1;i<=n;i++)
	{
		f>>x;
		V[x]++;
	}
	i=1;
	while(V[i]==0)min++,i++;
	i=7000;
	while(V[i]==0)max--,i--;
	for(i=min;i<=max;i++)
	{
		if(V[i]>=3)c+=V[i]*(V[i]-1)*(V[i]-2)/2;
		if(i>min&&V[i]!=0)
		{
			k=1;
			while(k<=i-min&&k<=max-i)
			{
				if(V[i-k]!=0&&V[i+k]!=0)
				{
					c+=V[i-k]*V[i+k];
				}
				k++;
			}
		}
	}
	g<<c;	
	f.close();
	g.close();
	return 0;
}