enter the x value and return the corresponding f (x) value

''' There is a function f (x) defined on natural number, which is defined as 
follows:
If x < 5, then f (x) = x;
If 5 < = x < 15, then f (x) = x + 6;
If x > = 15, then f (x) = x-6.
Try to write the function, enter the x value and return the corresponding f (x) 
value.
[input form] the input line represents the natural number x.
[output form] the output line represents the calculation result f (x). If the 
input data is illegal (e.g. negative integer), output "illegal input".
[sample input] 4
[sample output] 4
 '''
def math(x):
    if (x < 0):
        return "illegal input"
    elif(x<5):
        return x
    elif(5<=x and x<15):
        return x+6
    elif(x>=15):
        return x-6

x=int(input("Enter Number: "))
print(math(x))