class Dice{
	int[] side;
	double expectationValue = 0.0;
	
	public Dice(int i){
		side = new int[i];		
	}
	
	public void setSide(){
		for(int i = 0;i < side.length;i++){
			side[i] = i+1;
		}
		
		for(int i = 0;i < side.length;i++){
			expectationValue += (side[i] * (1 / (double)side.length));
		}
	}
}

public class No_57{
	public static void main(String[] args){
		java.util.Scanner sc = new java.util.Scanner(System.in);
		int n = sc.nextInt();
		
		Dice dice = new Dice(6);
		dice.setSide();
		
		System.out.println(dice.expectationValue * n);
	}
}