#include<stdio.h>
int v[9001],aux[7001][9001];
int main()
{
freopen("medie.in","r",stdin);
freopen("medie.out","w",stdout);
int n,i,j,t=0,k;
scanf("%d",&n);
for(i=1;i<=n;i++)
	{
		scanf("%d",&v[i]);
		aux[v[i]][0]=1;
		aux[v[i]][i]=1;
	}

for(i=1;i<=n;i++)
	for(j=1;j<=n;j++)
	   if(j!=i)
	   if(aux[2*v[i]-v[j]][0]==1)
		for(k=j+1;k<=n;k++)
		    if(k!=i)
			t+=aux[2*v[i]-v[j]][k];
printf("%d",t);
return 0;
}