"""
ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 
4 digits or exactly 6 digits.

If the function is passed a valid PIN string, return true, else return false.

Examples
"1234"   -->  true
"12345"  -->  false
"a234"   -->  false
"""


def validate_pin(pin):
    return len(pin) in (4, 6) and pin.isnumeric()
