PROGRAMMING:Venue arrangement

Title Source: algorithm design and analysis by Wang Xiaodong
Suppose there are enough venues for a batch of activities, and you want to use as few venues as possible. Design an effective
Greedy algorithm（ This problem is actually a famous graph coloring problem. If each activity is regarded as a part of a graph
Vertices, incompatible activities are connected by edges. Make adjacent vertices have the minimum coloring number of different colors, corresponding to the minimum number of colors to be found
Number of venues.)
###Input format:
The first line has a positive integer k, which means that there are k activities to be arranged.
In the next K rows, each row has 2 positive integers, representing the start time and end time of K activities to be scheduled. time
Minutes starting at 0.
###Output format:
Output the minimum number of venues.
###Input example:
```in
five
1 23
12 28
25 35
27 80
36 50
```
###Output example:
The corresponding output is given here. For example:
```out
three
```







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