#include<bits/stdc++.h>
using namespace std;
int d,c,dc=1,j=0;
int main(){
    cin>>d;
    for(int i=1;i<=d;i++){
        c+=dc;
        j++;
        if(j==dc){
            dc++;
            j=0;
        }
    }
    cout<<c;
    return 0;       
}
/**************************************************************
	Problem: 1520
	User: chenmingyu
	Language: C++
	Result: Accepted
	Time:39 ms
	Memory:2072 kb
****************************************************************/