#include<bits/stdc++.h>
using namespace std;
int a(int n){
  int r;
  if(n==1) r=1;
  else r=a(n-1)+n-1;
  return r;
  } 
int main(){
  int i,s=0;
  for(i=1;;i++){
  s=a(i)+s;
  if(s>=5000) break;
  } 
  cout<<s;
  return 0;
}
/**************************************************************
	Problem: 1146
	User: wtq001
	Language: C++
	Result: Accepted
	Time:4 ms
	Memory:2072 kb
****************************************************************/