题目描述
You have a circle of length C, and you are placing N arcs on it. Arc i has length L_i.
Every arc i is placed on the circle uniformly at random: a random real point on the circle is chosen, then an arc of length L_i centered at this point appears.
Note that the arcs are placed independently. For example, they may intersect or contain each other.
What is the probability that every real point of the circle will be covered by at least one arc? Assume that an arc covers its ends.
Constraints
- $2 \leq N \leq 6$
- $2 \leq C \leq 50$
- $1 \leq L_i < C$
- All input values are integers.
Input
Input is given from Standard Input in the following format:
N C
L_1 L_2 ... L_N
Output
Print the probability that every real point of the circle will be covered by at least one arc. Your answer will be considered correct if its absolute error doesn't exceed 10^{-11}.
Examples
Input
2 3
2 2
Output
0.3333333333333333
Input
4 10
1 2 3 4
Output
0.0000000000000000
Input
4 2
1 1 1 1
Output
0.5000000000000000
Input
3 5
2 2 4
Output
0.4000000000000000
Input
4 6
4 1 3 2
Output
0.3148148148148148
Input
6 49
22 13 27 8 2 19
Output
0.2832340720702695
输入格式
见题面
输出格式
见题面