1416  Running
Time Limit : 1 Second
Memory Limit : 128 MB
Submission: 443
Solved: 81
 Description
xiaoY walked on the ZiJing road of HUST, Suddenly he got a message which tell him Mr HH want check homework today. Without hesitation, xiaoY began running faster and faster to Mr HH's class, suppose xiaoY stood at X0, and ZiJing road can be considered as a horizontal line, Mr HH's class is at the end of this road.
Now give you X0, xiaoY's initial speed V0, xiaoY’s acceleration a0, and the end of the road X1.
Just calculate how much time xiaoY need to catch the class!
 Input
Multiple cases.
Each case is described with 4 integers
X0 V0 a0 X1
0 0 0 0 indicates end of input!
 Output
A real number rounded to 2nd digital after decimal point
 sample input

0 0 1 2 0 0 0 0
 sample output

2.00
 hint
 source
 He Jian, HUST Campus 2010, Preliminary