1176  Knots
Time Limit : 1 Second
Memory Limit : 128 MB
Submission: 239
Solved: 175
 Description
 An even number N of strands are stuck through a wall. On one side of the wall, a girl ties N/2 knots between disjoint pairs of strands. On the other side of the wall, the girl's groomtobe also ties N/2 knots between disjoint pairs of strands. You are to find the probability that the knotted strands form one big loop (in which case the couple will be allowed to marry).
For example, suppose that N = 4 and you number the strands 1, 2, 3, 4. Also suppose that the girl has created the following pairs of strands by tying knots: {(1, 4), (2,3)}. Then the groomtobe has two choices for tying the knots on his side: {(1,2), {3,4)} or {(1,3), (2,4)}.
 Input
 The input file consists of one or more lines. Each line of the input file contains a positive even integer, less than or equal to 100. This integer represents the number of strands in the wall.
 Output
 For each line of input, the program will produce exactly one line of output: the probability that the knotted strands form one big loop, given the number of strands on the corresponding line of input. Print the probability to 5 decimal places.
 sample input

4 20
 sample output

0.66667 0.28377
 hint
 source
 Rocky Mountain 2006