#include<stdio.h>
FILE*f=fopen("medie.in","r");
FILE*g=fopen("medie.out","w");
long n,a[9001], v[9001], b[9101];
void read()
	{
	fscanf(f,"%ld",&n);
	for(int i=1;i<=n;++i) fscanf(f,"%ld",&a[i]);
	}
/*void sort()
	{
	int i,ok;
	int aux;
	do
		{
		ok=1;
		for(i=1;i<n;++i) if (a[i]>a[i+1]) {ok=0;aux=a[i]; a[i]=a[i+1]; a[i+1]=aux;}
		}
	while(ok==0);
	}
  */
void form_v()
	{
	long i,j;
	for(i=1;i<n;++i)
		{
		b[a[i]]++;
		for(j=i+1;j<=n;++j)
			if((a[i]+a[j])%2==0) v[(a[i]+a[j])/2]++;
		}
	b[a[n]]++;
	}
void calcul()
	{
	long i,sol=0;
	for(i=1;i<=n;++i)
		if(v[a[i]]!=0) sol+=v[a[i]]-b[a[i]]+1;

	 fprintf(g,"%ld",sol);
	 }
int main()
	{
	read();
	form_v();
	calcul();
	return 0;
	}