Author:  A. Klenin  Time limit:  1 sec  
Input file:  input.txt  Memory limit:  256 Mb  
Output file:  output.txt 
Young physicist Masha studies lasers. Masha wants to create a simplest 2dimensional waveguide, consisting of two very long parallel mirrors, one coinciding with the Ox axis, and another located h millimeters above the first.
Masha's laser is located at coordinates (0, y) and emits light in direction (1, −1). Masha wants to hit a target located at coordinates (x_{t}, y_{t}) by choosing a good value for h. All coordinates are in millimeters.
Note that laser itself must fit into the waveguide, so h ≥ y. Both mirrors and laser are perfect, so light is always fully reflected at 45 degrees angle.
Input file contains three integers y x_{t} y_{t} — coordinates of laser and target.
Output file must contain a single integer — value of h. If there are several solutions, output the smallest one. If there are no solutions, output −1.
1 ≤ y, x_{t}, y_{t} ≤ 10^{9}
No.  Input file (input.txt ) 
Output file (output.txt ) 

1 


2 


3 

