#include<stdio.h>
long long int n,i,st,dr,a,c[7002],sol;
int main()
{
	FILE *f,*g;
	f=fopen("medie.in","r");
	g=fopen("medie.out","w");
	fscanf(f,"%lld",&n);
	for(i=1;i<=n;i++)
	{fscanf(f,"%lld",&a);c[a]=c[a]+1;}
	for(i=1;i<=7000;i++)
	if(c[i])
	{ sol+=(c[i]*(c[i]-1)*(c[i]-2))/2;
	  st=i-1;dr=i+1;
	  while(st>=1&&dr<=7000)
	  { sol+=c[st]*c[i]*c[dr];
	    st--;dr++;
	  }
	}
	fprintf(g,"%lld\n",sol);
	fcloseall();
	return 0;
}