#include<stdio.h>

long nr[7001],sol,j,i,n;
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]);
for(i=1;i<=n;i++)
  for(j=i+1;j<=n;j++)
    {
    if((v[i]+v[j])%2==0)
      nr[(v[i]+v[j])/2] ++;
    if(v[i] == v[j]) sol-=2;
    }
for(i=1;i<=n;i++)
  sol+=nr[v[i]];
printf("%ld",sol);




return 0;
}