#include <iostream>
using namespace std;
int main(){
    int n,i,s = 0,c = 0,k = 1;
    cin>>n;
    for(i = 1;i <= n;i++){
        s = s + k;
        c++;
        if(c == k){
            c = 0;
            k++;
        }
    }
        
    cout<<s<<endl;
        
}
/**************************************************************
	Problem: 1520
	User: linzihang
	Language: C++
	Result: Accepted
	Time:50 ms
	Memory:2072 kb
****************************************************************/