# Program 18: Prime Check
def main():
    num = int(input("Enter number: "))

    flag = 0

    for i in range(2, num):
        if num % i == 0:
            flag = 1

    if flag == 0 and num > 1:
        print("Prime number")
    else:
        print("Not prime")

main()

Enter number:  23


Prime number

# Program 19: GCD
def main():
    a = int(input("Enter first number: "))
    b = int(input("Enter second number: "))

    while b != 0:
        temp = b
        b = a % b
        a = temp

    print("GCD is:", a)

main()

Enter first number:  7
Enter second number:  9


GCD is: 1

# Program 20: LCM
def main():
    a = int(input("Enter first number: "))
    b = int(input("Enter second number: "))

    if a > b:
        greater = a
    else:
        greater = b

    while True:
        if greater % a == 0 and greater % b == 0:
            print("LCM is:", greater)
            break
        greater += 1

main()

Enter first number:  6
Enter second number:  9


LCM is: 18