#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("nrtri.in");
ofstream g("nrtri.out");

int a[860],n,nrsol;

void Cautbin(int x1, int x2)
{
int sum,ps,pd,mid;
sum=a[x1]+a[x2];
ps=max(x1,x2);
pd=n;

while (ps<=pd)
{
    mid=(ps+pd)/2;
    if (a[mid]>sum) pd=mid-1;
    if (a[mid]<=sum) ps=mid+1;
}
mid=(ps+pd)/2;
if (a[mid]==sum && a[mid+1]==sum) mid=mid+1;
nrsol=mid-max(x1,x2)+1;

}



int main()
{
    int i,j;
f>>n;
for (i=1;i<=n;i++)
{
    f>>a[i];
}
sort (a+1,a+n+1);
for (i=1;i<=n-2;i++)
{
    for (j=i+1;j<=n-1;j++)
    {

    Cautbin(i,j);

    }
}
g<<nrsol;


    return 0;

}