#include <cstdio>
#include <algorithm>
int nr,n,i,j,s,a[801];
using namespace std;
int cautare_binara(int x, int y)
{
    int st=y+1;
    int dr=n;
    int mij=(st+dr)/2;
    while (a[mij]!=x && st<=dr)
    {
        if (a[mij]>x) dr=mij-1;
        else st=mij+1;
        mij=(st+dr)/2;
    }
    if (a[mij]==x) return mij;
    else return dr;
}
int main()
{
    freopen("nrtri.in","r",stdin);
    freopen("nrtri.out","w",stdout);
    scanf("%d\n",&n);
    for (i=1; i<=n; i++) scanf("%d ",&a[i]);
    sort(a+1,a+n+1);
    for (i=1; i<=n-2; i++)
    {
        for (j=i+1; j<=n-1; j++)
        {
            s=a[i]+a[j];
            if (cautare_binara(s,j)!=j) nr=nr+cautare_binara(s,j)-j;
        }
    }
    printf("%d ",nr);
    return 0;
}