Given a tree represented as undirected graph. Count the number of nodes at given level l. It may be assumed that vertex 0 is root of the tree.
Input : 7
0 1
0 2
1 3
1 4
1 5
2 6
2
Output : 4
Input : 6
0 1
0 2
1 3
2 4
2 5
2
Output : 3