#include <iostream>
#include <fstream>
#include <algorithm>
#define NMAX 805
using namespace std;

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

int v[NMAX],n,i,j,st,dr,sum,val,ans,t,mij;

int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i];
    sort(v+1,v+n+1);
    for(i=1;i<=n-2;i++)
        for(j=i+1;j<=n-1;j++)
            {
                st=j+1;
                dr=n;
                sum=v[i]+v[j];
                t=0;
                while(st<=dr)
                {
                    mij=(st+dr)/2;
                    if (sum<v[mij]) dr=mij-1;
                    else if (sum>v[mij]) st=mij+1;
                    else
                    {
                        val=mij;
                        while(v[val]==v[mij]) val++;
                        ans+=(val-j-1);
                        t=1;
                        break;
                    }
                }
                if (t==0) ans+=(dr-j);
            }
    g<<ans;

    return 0;
}