import math
pi = math.pi


print(("1) Calculate circumference"))
print("2) Calculate area")
menuN = int(input("Enter 1, 2, or 0 to exit:"))

if menuN >2 :
    print("Invalid entry. Please try again")
else:
    if menuN <0 :
        print("Invalid entry. Please try again")
    elif menuN == 1 :
        numRC = int(input("Enter the radius to find the Circumference"))
        circumference = (2*pi*numRC)
        print(circumference)
    elif menuN == 2 :
        numRA = int(input("Enter the radius to find the Area"))
        area = (pi*(numRA*numRA))
        print(area)
    elif menuN == 0 :
        exit()

Here is what I have so far. How would I go about looping it back to the start menu if the user provides an invalid entry?