#include <stdio.h>

using namespace std;

FILE *f,*s;

long int m,n,i,j,v1[10000],v2[20000];
long long int rez;

int main()
{
	f=fopen("medie.in","r");
	s=fopen("medie.out","w");
	
	fscanf(f,"%ld\n",&n);
	
	for(i=1;i<=n;i++)
		fscanf(f,"%ld\n",&v1[i]);
	
	for(i=1;i<n;i++)
	{
		for(j=i+1;j<=n;j++)
		{
			m=v1[i]+v1[j];
			
			v2[m]++;
			
			if(m==v1[i]*2)
				rez--;
		}	
	}	
	
	for(i=1;i<=n;i++)
		rez+=v2[v1[i]*2]--;

	fprintf(s,"%lld\n",rez);
	
	fclose(s);
	
	return 0;
}