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