Summation of Reciprocals of Consecutive Products

Recall that when Gauss was young, he derived the formula for the sum of the first n positive integers,

$$\sum_{i=1}^n i = \frac{n\times(n+1)}{2}.$$

Similarly, LT once computed

$$\sum_{i=1}^{n-1} \frac{1}{i\times(i+1)} = 1 - \frac{1}{n},$$

Now, as you are still a young prodigy, your task is to compute the following sum:

$$\sum_{i=1}^n \frac{1}{\prod_{j=i}^{i+m-1}j} = S,$$

where the expression

$$\prod_{j=i}^{i+m-1}j$$

denotes the product of m consecutive integers starting from i.

Print the value of S in fixed-point notation with exactly 6 decimal places.

inputFormat

The input consists of a single line with two positive integers n and m separated by a space.

outputFormat

Output the computed value of S with exactly 6 digits after the decimal point.

sample

1 1

1.000000