#include<fstream.h>
v[7001];
int main()
{
int n,i,j,k,S,x;
ifstream f("medie.in");
ofstream g("medie.out");
f>>n;
S=0;
for(i=1;i<=n;i++){f>>v[i];
		  v[x]++;}
		  for(i=1;i<=7000;i++)
		  {k=v[i];
		  S+=(k-2)*(k-1)*k/2;
		  j=i-1;
		  k=i+1;
		  while(j>=1 && k<=7000)
		  {if(v[j]>0 && v[k]>0) S+=v[j]*v[i]*v[k];
		  j--;
		  k++;
		  }
		  }
		  g<<S;
		  f.close();
		  g.close();
		  return 0;
                  }