print("Welcome to the Power Bill Calculator")


# Input number of units consumed

units_input = input("Enter the number of units consumed: ")


# Check if the units input is valid

if units_input.replace('.', '', 1).isdigit():
    units = float(units_input)
    if units < 0:
        print("Error: Units consumed cannot be negative.")
    else:
        # Input rate per unit
        rate_input = input("Enter the rate per unit: ")

        # Check if the rate per unit input is valid
        if rate_input.replace('.', '', 1).isdigit():
            rate_per_unit = float(rate_input)
            if rate_per_unit < 0:
                print("Error: Rate per unit cannot be negative.")
            else:
                # Calculate the power bill
                total_bill = units * rate_per_unit

                # Display the result
                print(f"Total Power Bill: ${total_bill:.2f}")
        else:
            print("Error: Invalid input for rate per unit.")
else:
    print("Error: Invalid input for units consumed.")