#include<stdio.h>
#include <algorithm>


using namespace std;

int n,a[1000],nr;

void citire()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
      scanf("%d",&a[i]);
    sort(a+1,a+n+1);
}

void solve()
{
    for(int i=1;i<=n-2;i++)
      for(int j=i+1;j<n;j++)
        {int v=a[i]+a[j];
         int st=j+1,mij,dr=n;
        while(st<=dr)
         {
          mij=(st+dr)/2;
          if(a[mij]==v)
            break;
           else
             if(a[mij]<v)
               st=mij+1;
               else
                dr=mij-1;
         }
        if(a[mij]==v)
           nr=nr+mij-j;
          else
           if(dr<st&&dr>j)
              nr=nr+dr-j;
              else
                if(dr>st&&st>j)
                   nr=nr+st-j;

       }
    printf("%d",nr);
}

int main()
{

    freopen("nrtri.in","r",stdin);
    freopen("nrtri.out","w",stdout);
    citire();
    solve();

return 0;
}