A microwave accepts from 1 to 4 digits for the cook time. If the time entered is 1 or 2 digits, the time is interpreted as seconds. If the time entered is 3 or 4 digits, it is interpreted as MM:ss (i.e. minutes and seconds). The time entered is guaranteed to be between 1 and 9999. It is valid to enter values of 60 or more for each MM and ss. Return a string of the format "MM:ss" representing the cook time. For MM and ss values that are less than 10 use the value with a leading 0.