#include <bits/stdc++.h>
#define din cin
#define dout out
#define pi 3.14159265359
#define sw(x,y) x^=y,y^=x,x^=y
#define bmin(a,b)((a<b)?(a):(b))
#define bmax(a,b)((a>b)?(a):(b))
#define bminify(a,b)a=bmin(a,b)
#define bmaxify(a,b)a=bmax(a,b)
#define forq(i,ii,n)for(i=ii;i<n;i++)
using namespace std;
typedef long long ll;
ifstream in("medie.in");
ofstream out("medie.out");
int n,a[9999],i,j,s,k;
int main()
{
in>>n;
while(n--)in>>i,++a[i];
for(i=1;i<7001;i++)
{
    j=bmin(i-1,7000-i),s+=a[i]*(a[i]-1)*(a[i]-2)/2;
    for(k=1;k<=j;k++)s+=a[i]*a[i-k]*a[i+k];
}
out<<s;
}