MELE3 - MELE3

Solitaire has N elevators. Each elevator are connecting exactly two floors and it does not stop on the floors between that two floors The speed of all the elevators are the same, 5 seconds to pass one floor.

On the beginning, each elevator is in its lower position and they are starting cruising to the upper floor. After some elevator come to its upper position, it immediately starts to go back to its lower position, and so on...

Mirko is on the first (the lowest) floor and he wants as quick as possible come to the top of the solitaire. He can change elevators only on the floors that are common to both elevators, and if the other elevator is in that moment on that floor, that change does not take any time.

Write a program that will calculate minimal time in which Mirko can get to the top of the solitaire.

Input

In the first line of the input file there are two integers K and N, separated with space, number of floors in solitaire and number of elevators, 2 ≤ K ≤ 1000, 1 ≤ N ≤ 50000.

In each of the next N lines there are description of one elevator, two integers A and B, separated with space, 1 ≤ A < B ≤ K, means that elevator is travelling between floors A and B.

There are no two different elevators that travels between same floors.

Note: input data will guarantee that solution will always exists.

Output

In the only line of output file write minimal time (in seconds) from the text above.

Sample

Input:
10 4
1 5
5 10
5 7
7 10

Output:
45
Input:
10 3
1 5
3 5
3 10

Output:
105
Input:
20 5
1 7
7 20
4 7
4 10
10 20

Output:
150

Added by:psetter
Date:2009-04-08
Time limit:1s
Source limit:50000B
Memory limit:1536MB
Cluster: Cube (Intel G860)
Languages:All except: ERL JS-RHINO NODEJS PERL6 VB.NET
Resource:COI 03

hide comments
2023-04-11 00:12:08
Despite what the problem implies, You shouldn't use
freopen("liftovi.in", "r", stdin);
freopen("liftovi.out", "w", stdout);
or any method that outputs to file, just submit normally.

[Simes]: fixed, thanks.

Last edit: 2023-04-11 08:01:24
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