#include<stdio.h>

long nr2[7001],sol,j,i,n;
int nr[7001];
int v[9001];

int main()
{
freopen("medie.in","r",stdin);
freopen("medie.out","w",stdout);
scanf("%ld",&n);
for(i=1;i<=n;i++)
  {
  scanf("%d",&v[i]);
  nr[v[i]]++;
  }
for(i=1;i<=7000;i++)
  if(nr[i])
    {
    for(j=i+2;j<=7000;j+=2)
      nr2[(i+j)/2]+=nr[i]*nr[j];
    nr2[i] += (nr[i]*(nr[i]-1))/2;
    sol-=nr[i]*(nr[i]-1);
    }



for(i=1;i<=n;i++)
  sol+=nr2[v[i]];
printf("%ld",sol);




return 0;
}