You have a computer literacy course in your university. In the computer system, the login/logout records of all PCs in a day are stored in a file. Although students may use two or more PCs at a time, no one can log in to a PC which has been logged in by someone who has not logged out of that PC yet.
You are asked to write a program that calculates the total time of a student that he/she used at least one PC in a given time period (probably in a laboratory class) based on the records in the file.
The following are example login/logout records.
* The student 1 logged in to the PC 1 at 12:55
* The student 2 logged in to the PC 4 at 13:00
* The student 1 logged in to the PC 2 at 13:10
* The student 1 logged out of the PC 2 at 13:20
* The student 1 logged in to the PC 3 at 13:30
* The student 1 logged out of the PC 1 at 13:40
* The student 1 logged out of the PC 3 at 13:45
* The student 1 logged in to the PC 1 at 14:20
* The student 2 logged out of the PC 4 at 14:30
* The student 1 logged out of the PC 1 at 14:40
For a query such as "Give usage of the student 1 between 13:00 and 14:30", your program should answer "55 minutes", that is, the sum of 45 minutes from 13:00 to 13:45 and 10 minutes from 14:20 to 14:30, as depicted in the following figure.