#include <fstream>
using namespace std;

int N,v[801];

ifstream f("nrtri.in");
ofstream g("nrtri.out");

inline int cb(int i,int j)
{
    int st,dr,mij,rez=0;
    for (st = 1,dr = N;st <= dr; )
    {
        mij = st + ((dr - st) >> 1);
        if (i + j >= v[mij]) rez=mij-2,st=mij + 1;
        else dr=mij - 1;
    }
    return rez;
}
void solve()
{
    int i,j,rez=0;
    f >> N;
    for (i=1;i<=N;i++) f >> v[i];
    for (i=1;i<=N-1;i++)
      for (j=i+1;j<=N;j++)
         rez+=cb(i,j);
    g << rez;
}
int main()
{
    solve();
    return 0;
}