At Fred's Automotive works the arrivals of customes at the Desk is timed.Let X= the number of minutes between successive disgruntled customers. It's found that cumulative distribution function of X is:

F(x)= 0 if x<1

F(x)=0.3 if 1<=x<3

F(x)=0.4 if 3<=x<4

F(x)=0.45 if 4<=x<6

F(x)=0.60 if 6<=x<12

F(x)=1 if 12<=x

What is the pmf of X?

Many thanks to everyone who try to help.