#include<bits/stdc++.h>
using namespace std;
int main(){ 
    int k;
    cin>>k;
    int ans=0,cur=1,cnt=0;
    for (int i=1;i<=k; ++i){
        ans+=cur;
        if (++cnt==cur){
            ++cur;
            cnt=0;
        }
    }
    cout<<ans;
    fclose(stdin);
    fclose(stdout);
}
/**************************************************************
	Problem: 2340
	User: wangyichen2
	Language: C++
	Result: Accepted
	Time:47 ms
	Memory:2072 kb
****************************************************************/