Balanced lineup

Farmer Keller (who's standing in for FR this problem) 
has decided to take a family portrait of some (maybe
all) of the cows. In order to take the portrait, FK has arranged
all N (1 <= N <= 50,000) cows in a straight line. Each cow is labeled
with its x coordinate (range: 0..1,000,000,000) on the line as well
as its breed (0 or 1).

Over the years, FK has done some crazy things, and this activity
is no exception. He will only photograph one group of cows, and
that group must be "balanced". A contiguous group of cows is
"balanced" if it contains the same number of cows from each of the
two breeds.

Determine the widest expanse of cows that might have to fit in one
photograph. The length of a balanced group is the difference in x
values between the left-most and right-most cows in the group.

At least one cow of each breed appears in the input, and no two
cows share the same x coordinate.

PROBLEM NAME: balance.X

INPUT FORMAT:

* Line 1: A single integer: N

* Lines 2..N+1: Each line contains two space-separated integers that
        describe a single cow containing respectively: a breed ID and
        an x coordinate

SAMPLE INPUT:

7
0 11
1 10
1 25
1 12
1 4
0 13
1 22

INPUT DETAILS:

There are 7 cows, five of breed 1 and two of breed 0 arranged like
this:
            1                 1  0  1  0                          1        1
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

OUTPUT FORMAT:

* Line 1: A single integer specifying the size of the largest
        contiguous balanced group of cows.

SAMPLE OUTPUT:

11

OUTPUT DETAILS:

Cows #1 (at 11), #4 (at 12), #6 (at 13), and #7 (at 22) form a
contiguous balanced group occupying 22-11=11 units of length on the
number line:

                                 <-------- balanced group -------->
            1                 1  0  1  0                          1        1
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25