#include<stdio.h>
int main()
{
    int x;
    scanf("%d",&x);
  
    if(x==1)
        printf("%.1lf",5.0);
    if(x>=2&&x<=5)
        printf("%.1lf",4.5*x);
    if(x>=6&&x<=10)
        printf("%.1lf",4.0*x);
    if(x>10)
        printf("%.1lf",3.5*x);
        return 0;
}

/**************************************************************
	Problem: 1717
	User: admin
	Language: C
	Result: Accepted
	Time:60 ms
	Memory:1144 kb
****************************************************************/