#include<fstream.h>
long a[50001],N,max=0;
void citire(){
ifstream fin("medie.in");
fin>>N;
long x;
for (int i=0;i<N;i++){
    fin>>x;
    if (x>max)
      max=x;
    a[x]++;}
fin.close();
}
long long numar(){
long long nr=0,S;
for (long i=1;i<=max;i++) {
    if (a[i]>2){
       S=0;
       for (int u=2;u<=N-1;u++)
	   S+=N-u;
       nr+=a[i]*S;   }
  for (long j=i+1;j<=2*i;j++)
	   nr+=a[j]*a[2*i-j]*a[i];}
return nr;
}
int main(){
citire();
ofstream fout("medie.out");
fout<<numar();
fout<<"\n";
fout.close();
return 0;
}