include <bits/stdc++.h>

using namespace std; int main (){

int k;
cin>>k;
int a=0;
int n=1,days=1;
while(days<=k){
	for(int i=1;i<=n;i++){
		a=a+n;
		days++;
		if (days>k){
			break;
		}
	}
	n++;
}
cout<<a;
return 0;

}