Managing time... mtime.X

Ever the maturing businessman, FR realizes that he must
manage his time effectively. He has N jobs conveniently numbered
1..N (1 <= N <= 1,000) to accomplish (like milking the cows, cleaning
the barn, mending the fences, and so on).

To manage his time effectively, he has created a list of the jobs
that must be finished. Job i requires a certain amount of time T_i
(1 <= T_i <= 1,000) to complete and furthermore must be finished
by time S_i (1 <= S_i <= 1,000,000). FR starts his day at
time t=0 and can only work on one job at a time until it is finished.

Even a maturing businessman likes to sleep late; help FR
determine the latest he can start working and still finish all the
jobs on time.

PROBLEM NAME: mtime.X

INPUT FORMAT:

* Line 1: A single integer: N

* Lines 2..N+1: Line i+1 contains two space-separated integers: T_i
        and S_i

SAMPLE INPUT:

4
3 5
8 14
5 20
1 16

INPUT DETAILS:

There are 4 jobs to do, which take 3, 8, 5, and 1 units of
time, respectively, and must be completed by time 5, 14, 20, and
16, respectively.

OUTPUT FORMAT:

* Line 1: The latest time that it is possible to start working or -1 if
          it's not possible to finish all the jobs on time.

SAMPLE OUTPUT:

2

OUTPUT DETAILS:

Farmer Ran must start the first job at time 2. Then he can do
the second, fourth, and third jobs in that order to finish on time.