#include<stdio.h>
int v[9001];
int w[7001];
long cont;
int main()
{int n,x,i,j,ok;
 FILE *f=fopen("medie.in","r");
 FILE *g=fopen("medie.out","w");
 fscanf(f,"%d",&n);
 for(i=1;i<=n;i++)
  {fscanf(f,"%d",&x);
   v[i]=x;
   w[x]++;
  }
 cont=0;
 for(i=1;i<=n;i++)
  for(j=(i+1);j<=n;j++)
   if(v[i]==v[j])
     cont+=(w[v[i]]-2);
    else
     {ok=v[i]+v[j];
      if(ok%2==0)
	{ok=ok/2;
	 cont=cont+w[ok];
	}
     }
 fprintf(g,"%d",cont);
 fprintf(g,"\n");
return 0;
}