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ZOJ Problem Set - 1619
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Time Limit: 2 Seconds      Memory Limit: 65536 KB

There are N friends who has bought one present each. They mix their presents together and then each person takes one randomly. What is the probability that exactly M people got their own present.


Input

Each line is one case and contains two integers N (0 < N < 100), M (0 <= M <= N).


Output

For each case, output the result in one line.

The result are exact to 8 decimal places.


Sample Input

3 1
4 2


Sample Output

0.50000000
0.25000000



Author: ZHANG, Zheng
Source: Zhejiang University 2003 Summer Camp Qualification Contest
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