a?

#include<iostream>
using namespace std;

int main(){
	int n;
    cin>>n;
    int i = 1;
    int k = 1;
    int sum = 0;
    while(i<=n){
        int j = 1;
		while(j<=k and i<=n){
			sum = sum + k;
            j += 1;
            i += 1;
        }
        k += 1;
	}
	cout<<sum;
    return 0;
}