317. Fast Ride

Time limit per test: 0.25 second(s)
Memory limit: 65536 kilobytes
input: standard
output: standard

Is there a Russian who does not like a fast ride?! Royal messenger should get from point A to point B as quickly as possible. Point A has coordinate 0 on the Ox axis. Point B has positive coordinate B on the Ox axis. Of course, the messenger will use horses to get to the destination point. The messenger is not allowed to travel by foot because of an importance of his mission and security reasons.

There are N stables on the straight line from A to B. The i-th stable has Mi horses inside. Each horse is characterized by its speed vj and maximum distance it can gallop dj (after that it falls exhausted). The messenger can change the horse reaching or passing a stable to any horse from that stable without losing time.

Your task is to find the minimum time the messenger needs to get to the point B.

The first line of the input contains two integer numbers B and N (1 ≤ B ≤ 108; 1 ≤ N ≤ 5000), where N is the number of stables. Descriptions of the stables follow. The first line of each description contains two integer numbers Xi, Mi (0 ≤ Xi ≤ 108; 0 < Mi) — the position of the stable and the number of horses in this stable respectively. The following Mi lines contain the description of the horses. Each description of a horse is a pair of integer numbers vj, dj (1 ≤ vj ≤ 108; 1 ≤ dj ≤ 108). The total number of horses in all stables is no more than 105. It is possible that two or more stables have the same position.

Write the minimum time which will take the messenger to get to the point B to the output. The answer should be written with no less than 3 digits after the decimal point. If the solution does not exist, write -1 to the output.

sample input
sample output
10 3
5 2
1 100
10 3
0 1
1 50
8 1
2 3

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