order intervals in a data.frame

I would like to know how can I order a data.frame with increasing the dat$Interval (dat$Interval is a factor). There is an example below.

Original data.frame:

dat

                 Interval               Number_reads
                  0-100                         685
               200-300                         744
               100-200                        1082
               300-400                        4213

Desired_dat:

                 Interval               Number_reads
                  0-100                          685
                100-200                       1082
                200-300                         744
                300-400                       4213