#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("nrtri.in");
ofstream fout("nrtri.out");
int n,v[3001],s,i,j,st,dr,sol,mid;
int main() {
    fin>>n;
    for (i=1;i<=n;i++)
        fin>>v[i];
    sort(v+1,v+n+1);
    for (i=1;i<=n-2;i++) {
        for (j=i+1;j<=n-1;j++) {
            s=v[i]+v[j];
            ///caut pe k in v intre pozitiile j+1 si n
            st=j+1;
            dr=n;
            while (st<=dr) {
                mid=(st+dr)/2;
                if (v[mid]>s)
                    break;
                if (v[mid]<s)
                    st=mid+1;
            }
            if (st<=dr)
                sol=sol+dr-j;
        }
    }
    fout<<sol;
    return 0;
}