Equivalent Sets

To prove two sets A and B are equivalent, we can first prove A is a subset of B, and then prove B is a subset of A, so finally we got that these two sets are equivalent.
You are to prove N sets are equivalent, using the method above: in each step you can prove a set X is a subset of another set Y, and there are also some sets that are already proven to be subsets of some other sets.
Now you want to know the minimum steps needed to get the problem proved.

The input file contains multiple test cases, in each case, the first line contains two integers N <= 20000 and M <= 50000.
Next M lines, each line contains two integers X, Y, means set X in a subset of set Y.

For each case, output a single integer: the minimum steps needed.

4 0
3 2
1 2
1 3

4
2

Case 2: First prove set 2 is a subset of set 1 and then prove set 3 is a subset of set 1.

2011 Multi-University Training Contest 1 - Host by HNU