#include <fstream>
#include <algorithm>

using namespace std;

ifstream fin("nrtri.in");
ofstream fout("nrtri.out");

const int NMAX = 800;

int v[NMAX + 1], n, n2;

int cautbin1(int x)
{
    int sol = 0;
    for(int i = n2; i > 0; i/=2)
    {
        if(sol + i <= n && v[sol + i] <= x)
        {
            sol += i;
        }
    }
    return sol;
}

int main()
{
    int c, p, aux;
    c = 0;
    fin >> n;
    for(int i = 1; i <= n; i++)
    {
        fin >> v[i];
    }
    for(int i = 1; i <= n; i++)
    {
        for (int j = i+1; j <= n; j++)
            if (v[i] > v[j])
            {
                aux = v[i];
                v[i] = v[j];
                v[j] = aux;
            }
    }
    for(int i = 2; i <= n; i += 2)
    {
        for(n2 = 1; n2 * 2 <= n; n2*=2)
        {
        }
        p = cautbin1(v[i - 1] + v[i]);
        c += p - i;
    }
    fout << c;
    return 0;
}