# Groups

Time Limit: 2000/1000 MS (Java/Others)

Memory Limit: 32768/32768 K (Java/Others)

## Description

After the regional contest, all the ACMers are walking alone a very long avenue to the dining hall in groups. Groups can vary in size for kinds of reasons, which means, several players could walk together, forming a group.
As the leader of the volunteers, you want to know where each player is. So you call every player on the road, and get the reply like “Well, there are Ai players in front of our group, as well as Bi players are following us.” from the ith player.
You may assume that only N players walk in their way, and you get N information, one from each player.
When you collected all the information, you found that you’re provided with wrong information. You would like to figure out, in the best situation, the number of people who provide correct information. By saying “the best situation” we mean as many people as possible are providing correct information.

## Input

There’re several test cases.
In each test case, the first line contains a single integer N (1 <= N <= 500) denoting the number of players along the avenue. The following N lines specify the players. Each of them contains two integers Ai and Bi (0 <= Ai,Bi < N) separated by single spaces.
Please process until EOF (End Of File).

## Output

For each test case your program should output a single integer M, the maximum number of players providing correct information.

## Sample Input

3
2 0
0 2
2 2
3
2 0
0 2
2 2

## Sample Output

2
2
Hint
The third player must be making a mistake, since only 3 plays exist.


liuyiding

## Source

2012 ACM/ICPC Asia Regional Chengdu Online