#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;
}