Beat the Flash

Time Limit: 1 Second(s)    Memory Limit: 32 MB

Total Submission(s): 26   Accepted Submission(s): 11
Problem Description

XiaoMing is a normal person. One day he suddenly got magic speed power, that his speed increases t units each day(At the very beginning, his speed is a, and t is the time counting from the day he got the power). XiaoMing wants to know when he could be as fast as the flash once was(that their speed are exactly the same). The flash’s initial speed is b, and his speed increases in the same way like XiaoMing’s. So when is the earliest day that XiaoMing could have the same speed as the Flash had once?

Input

There are multiple cases. Each case contains two integers a and b(0<a,b<=10000), a and b has no certain relationship on size.(a could be bigger, smaller or equal to b).

Output

If XiaoMing could have a speed equal to the Flash had once, output the earliest day that XiaoMing achieves this goal, else if not, output -1. (The first beginning is regarded as the 0th day)

Sample Input
```2 4
```
Sample Output
```2
```
Source

2017绍兴市联赛