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