#include<fstream.h>
int v[7001];
int main ()
{float i,j,k,n,nr,x,s;
ifstream f("medie.in");
ofstream g("medie.out");
f>>n;
for(i=1;i<=7000;i++) v[i]=0;
for(i=1;i<=n;i++) {f>>x; v[x]++;}
s=0;
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;
}