// medie.cpp : Defines the entry point for the console application.
//

#include <stdio.h>

int e[10000];
int ap[10000];
char c[100000];
unsigned int aparitii=0;
int el,n,md;
void parsare()
{
	int x=0,nr=0;
	fread(c,1,100000,stdin);
	for(int i=0; c[i]!=EOF && c[i] ;i++)
		if(c[i]=='\n')
		{
			e[++nr]=x;
			ap[x]++;
			x=0;
		}
		else
			x=10*x+c[i]-'0';
	if(x)
	{
		e[++nr]=x;
		ap[x]++;
	}
}
int main()
{
	int i,j;
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d\n",&n);
	parsare();
	for(i=1;i<=n;i++)
		for(j=i+1;j<=n;j++)
			if((float)(e[i]+e[j])/2==(e[i]+e[j])>>1)
			{
				el=0;
				md=(e[i]+e[j])>>1;
				if(e[i]==md)
					el++;
				if(e[j]==md)
					el++;
				if(ap[md]-el>0)
					aparitii=aparitii+ap[md]-el;
			}

	printf("%d",aparitii);

	return 0;
}