#include<stdio.h>
#include<algorithm>  
using namespace std;
#define dim 9000
int a[dim],n,m,i,k,nr,j;
int b[7001];
float sol;
void read()
{
     scanf("%d",&n);
     for(i=1;i<=n;i++)
     {
                      scanf("%d",&a[i]);
                      b[a[i]]++;
                      }
     
}
void solve()
{
    for(i=1;i<n;i++)
    for(k=i+1;k<=n;k++)
    {
                       sol=(float)(a[i]+a[k])/2;
                       if((int)sol==sol)
                          if(b[(int)sol])
                          {
                                          nr+=b[(int)sol];
                                          if(a[i]==sol)
                                          nr--;
                                          if(a[k]==sol)
                                          nr--;
                           }}
}

 
int main ()
{
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    read();
    solve();
    printf("%d",nr);
    return 0;
}