Network 
Time limit:  1.00 s 
Memory limit:  512 MB 

Syrjälä's network consists of $n$ computers and $n$ connections between them. At the moment it is possible to send a message between any two computers.
Your task is to calculate the number of
essential connections: if such a connection is removed from the network, it is not possible to send a message between some two computers.
Input
The first input line contains an integer $n$: the number of computers. The computers are numbered $1,2,\ldots,n$.
After this, there are $n$ lines that describe the connections. Each line has two integers $a$ and $b$: there is a connection between computers $a$ and $b$.
There is at most one connection between two computers, and there are no connections from a computer to itself.
Output
Print one integer: the number of essential connections.
Example
Input:
7
1 2
2 3
2 4
4 5
2 5
5 6
6 7
Output:
4
Explanation: The essential connections are $(1,2)$, $(2,3)$, $(5,6)$ and $(6,7)$.
Subtask 1 (28 points)
Subtask 2 (72 points)