OJ Board

I think it has got to do with the limit of the strings being 20... if you track in certain order, some of them would give numbers which are beyond 20... like if you track for insert first, then you delete... then you might insert letters beyond the limit of 20 and then delete whatever you want to ...

Try this:
Input:

8 10
1 2
1 3
1 4
1 5
6 2
6 3
6 4
6 5
1 7
7 8
0 0

The output should be 2