P6933 [ICPC 2017 WF] Need for Speed

 Sheila is a student and she drives at ypical student car: it is old, slow, rusty, and falling apart. Recently,the needle on the speedometer fell off. She glued it back on, but she might have placed it at the wrong angle. Thus, when the speedometer reads $s$,her true speed is $s+c$, where $c$ is an unknown constant(possibly negative). Sheila made a careful record of a recent journey and wants to use this to compute $c$ . The journey consisted of $n$ segments. In the $i^{th}$ segment she traveled a distance of $d_{i}$ and the speedometer read $s_{i}$ for the entire segment. This whole journey took time $t$ . Help Sheila by computing $c$ . Note that while Sheila's speedometer might have negative readings, her true speed was greater than zero for each segment of the journey.

The first line of input contains two integers $n (1 \le n \le 1 000)$ , the number of sections in Sheila's journey, and $t (1 \le t \le 10^{6}),$ the total time. This is followed by $n$ lines, each describing one segment of Sheila's journey. The $i^{th}$ of these lines contains two integers $d_{i} (1 \le d_{i} \le 1 000)$ and $s_{i} (|s_{i}| \le 1 000)$ , the distance and speedometer reading for the $i^{th}$ segment of the journey. Time is specified in hours, distance in miles, and speed in miles per hour.

Display the constant $c$ in miles per hour. Your answer should have an absolute or relative error of less than $10^{−6}.$

Time limit: 1 s, Memory limit: 512 MB. spj provider:@[shenyouran](/user/137367).