#include<stdio.h>
struct doi{
	int i,j;
};
int v[9001];
doi aux[7001];
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]].i=1;
		aux[v[i]].j++;
	}

for(i=1;i<=n;i++)
	for(j=i+1;j<=n;j++)
	    if(v[i]%2==v[j]%2&&aux[(v[i]+v[j])/2].i==1)
	    {
		if(v[i]==v[j]&&aux[v[i]].j>=2)
			t+=aux[v[i]].j-2;
		else
			t++;
	    }
printf("%d",t);
return 0;
}