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