#include<fstream.h>
int v[9001];
int w[7001];
unsigned long cont;
int main()
{int n,x,i,j,ok,y;
 ifstream f("medie.in");
 ofstream g("medie.out");
 f>>n;
 for(i=1;i<=n;i++)
  {f>>x;
   v[i]=x;
   w[x]++;
  }
 f.close();
 cont=0;
 for(i=1;i<=n;i++)
  for(j=(i+1);j<=n;j++)
   {ok=v[i]+v[j];
    if(ok%2==0)
      {ok=ok/2;
       w[v[i]]--;
       w[v[j]]--;
       cont=cont+w[ok];
       w[v[i]]++;
       w[v[j]]++;
      }
   }
 g<<cont<<'\n';
 g.close();
return 0;
}