gradetable = {
    "90-100": "A",
    "80-89": "B",
    "70-79": "C",
    "60-69": "D",
    "0-59": "F"
}

def main():
    usercontinue = 'yes'
    while usercontinue == 'yes':
        try:
            score = int(input("What was the exam score?: "))

            if 90 <= score <= 100:
                grade = gradetable["90-100"]
            elif 80 <= score <= 89:
                grade = gradetable["80-89"]
            elif 70 <= score <= 79:
                grade = gradetable["70-79"]
            elif 60 <= score <= 69:
                grade = gradetable["60-69"]
            elif 0 <= score <= 59:
                grade = gradetable["0-59"]
            else:
                grade = 'invalid score'

            print(f"The letter score of score of the exam was {grade}")

        except ValueError:
            print("Invalid input. Please enter a number.")


        usercontinueprompt = input("Would you like to determine the letter grade of another score? (yes/no): ")
        usercontinueprompt = usercontinueprompt.lower() #Convert to lowercase once

        if usercontinueprompt == 'yes': #Corrected to assignment
            usercontinue = 'yes'
        elif usercontinueprompt == 'no':
            print("Thanks, have a great day!")
            usercontinue = 'no'  # Corrected typo and assignment
        else:
            print("That wasn't a valid selection. Try again")


if __name__ == "__main__":
    main()