Present

Time Limit: 1 Second    Memory Limit: 32768 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

Submit

Source: Zhejiang University 2003 Summer Camp Qualification Contest