convert hexadecimals to decimals

def main():
    # Prompt the user to enter a hex number
    hex = input("Enter a hex number: ").strip()

    decimal = hexToDecimal(hex.upper())
    if decimal == None:
        print("Incorrect hex number")
    else:
        print("The decimal value for hex number", hex,"is",decimal)


def hexToDecimal(hex):
    decimalValue = 0
    for i in range(len(hex)):
        ch = hex[i]
        if 'A' <= ch <= 'F' or '0' <= ch <= '9':
            decimalValue = decimalValue * 16 + hexCharToDecimal(ch)

        else:
            return None


    return decimalValue


def hexCharToDecimal(ch):
    if 'A' <= ch <= 'F':
        return 10 + ord(ch) - ord('A')
    else:
        return ord(ch) - ord('0')


main() # Call the main function