#include <stdio.h>

long long tort;
int freq[7000],med[7000],sir[9000],n,max;

void cit(FILE *f)
{
	int i;
	fscanf(f,"%i",&n);
	for(i=0;i<n;i++)
	{
		fscanf(f,"%i",&sir[i]);
		if(sir[i]>max)
			max=sir[i];
	}
}

void rez()
{
	int i,j,a;
	for(i=0;i<max;i++)
	{
		freq[sir[i]]++;
		for(j=i+1;j<max;j++)
			if(!((a=sir[i]+sir[j])%2))
				med[a/2]++;
	}
}

int main()
{
	int i;
	cit(fopen("medie.in","r"));
	rez();
	for(i=0;i<max;i++)
		tort+=med[sir[i]]-freq[sir[i]]+1;
	fprintf(fopen("medie.out","w"),"%lli\n",tort);
	return 0;
}