579_C. A Problem about Polyline
There is a polyline going through points (0, 0) – (x, x) – (2x, 0) – (3x, x) – (4x, 0) – ... - (2kx, 0) – (2kx + x, x) – .... 

We know that the polyline passes through the point (a, b). Find minimum positive value x such that it is true or determine that there is no such x.

Input

Only one line containing two positive integers a and b (1 ≤ a, b ≤ 109).

Output

Output the only line containing the answer. Your answer will be considered correct if its relative or absolute error doesn't exceed 10 - 9. If there is no such x then output  - 1 as the answer.

Examples

Input

3 1


Output

1.000000000000


Input

1 3


Output

-1


Input

4 1


Output

1.250000000000

Note

You can see following graphs for sample 1 and sample 3. 

<image> <image>
```python
a,b=(int(i) for i in input().split())
if (a<b):
    print('-1')
elif (a==b):
    print(a)
else:
    i1=(a+b)//(2*b);
    i2=(a-b)//(2*b)
    if i2 is not 0:
        p=(a+b)/2
        q=(a-b)/2
        print(min(p/i1,q/i2))
    else:
        print((a+b)/(2*i1))

```