probability - throwing a dice repeatedly so that each side appear once.

Pratt is given a fair die. He repeatedly
throw the die until he get
at least each number (1 to 6).

Define the random variable $X$ to be the total number of trials that
pratt throws the die. or
How many times he has to throw a die so that each side of the die appears at least once.
Determine the expected value $E(X)$.