#include <iostream>
using namespace std;
int main(){
long n,coin=1,sum=0;
cin >> n ;
int count = 1;
while(count <= n){
for(int j=1;j<=coin&&count <=n;j++){
sum = sum + coin;
count = count + 1;
}
coin = coin + 1;
}
cout << sum << endl;
return 0;
}