import random

# Programmer: Sriram Pemmaraju
# Date: Feb 10, 2012

# Version 3: moves the person at random, one step at a time, left or right,
# until the person reaches a barrier n or -n. Outputs the number of steps
# it took to reach the barrier

# This function takes in the value of the barrier, simulates a random
# walk that terminates on reaching the barrier, and returns the length
# of the simulated random walk
def randomWalk(n):
    location = 0 # tracks the person's current location
    length = 0 # tracks the length of the random walk

    # This moves the person until she reaches the barrier at n or -n
    while abs(location) != n:    
        step = random.randint(0, 1) # returns 0 or 1, each with prob. 1/2

        # Adjusts the random number to be either -1 or +1
        if step == 0:
            step = -1    
        location = location + step # updates location
        length =  length + 1

    return length

# This is the main part of the program (i.e., outside the function)
n = input("Enter the value of the barrier (a positive integer): ")

sum = 0 # track the total length of all simulated random walks
counter = 0
while counter < 100:
    sum = sum + randomWalk(n)
    counter = counter + 1
    
print float(sum)/100