#include<cstdio>
int main()
{
	int k,n=1,ans=0,j;
	scanf("%d",&k);
		for(j = 1;;j++){
			ans+=n;
			k--;
			if(k==0){printf("%d",ans);return 0;}
			if(j==n){
				j=1;n++;
			}
		}
}