#include<stdio.h>
#include<math.h>
int main()
{
 FILE *f,*g;             	
 f=fopen("fractii.in","r");
 long int a;

 fscanf(f,"%D",&a);
 long b;
 b=a*(a+1)/2-1;
 long int i=2;
 int c;
 float c1;
 c1=sqrt(a)+1;

 c=sqrt(a)+1;
 if(c1!=c)
   c++;
 for(;i<c;i++)
    b+=i;

 g=fopen("fractii.out","w");
 fprintf(g,"%d",b);
 fclose(f);
 fclose(g);
 return 0;
}