def collatz_sequence(n):
    """Generate the Collatz sequence for a given number n."""
    if n <= 0:
        raise ValueError("Input must be a positive integer.")
        
    sequence = [n]  # Start with the initial number
    
    while n != 1:
        if n % 2 == 0:
            n //= 2  # n is even
        else:
            n = 3 * n + 1  # n is odd
        sequence.append(n)
    
    return sequence

def main():
    try:
        number = int(input("Enter a positive integer: "))
        sequence = collatz_sequence(number)
        print(f"Collatz sequence for {number}:")
        print(" -> ".join(map(str, sequence)))
    except ValueError as e:
        print(e)

if __name__ == "__main__":
    main()