include<bits/stdc++.h>

using namespace std; int main(){

int k,xjb=1,zjb=0,xts=0,zts=0,total=1;
cin>>k;
while(zts<k){
	xts++;
	zjb+=total;
	zts++;
	if(xts==total)
	{
		total++;
		xts=0;
	}
}
cout<<zjb;

return 0; }