Bessie is in a 2D grid where walking is permitted only in directions parallel to
one of the coordinate axes. She starts at the point $(0,0)$ and wishes to reach
$(N,N)$ ($1\le N\le 10^9$). To help her out, there are $P$ ($1\le P\le 10^5$)
springboards on the grid. Each springboard is at a fixed point $(x_1,y_1)$ and
if Bessie uses it, she will land at a point
$(x_2,y_2)$.

Bessie is a progress-oriented cow, so she only permits herself to walk up or
right, never left nor down. Likewise, each springboard is configured to never go
left nor down. What is the minimum distance Bessie needs to walk?

SCORING:
Test cases 2-5 satisfy $P \le 1000$.Test cases 6-15 satisfy no additional constraints.

INPUT FORMAT:
The fist line contains two space-separated integers $N$ and $P$.

The next $P$ lines each contains four integers, $x_1$, $y_1$, $x_2$, $y_2$,
where $x_1 \le x_2$ and $y_1 \le y_2.$

All springboard and target locations are distinct.

OUTPUT FORMAT:
Output a single integer, the minimum distance Bessie needs to walk to reach
$(N,N)$.

SAMPLE INPUT:
3 2
0 1 0 2
1 2 2 3
SAMPLE OUTPUT:
3

Bessie's best path is:

Bessie walks from (0,0) to (0,1) (1 unit).

Bessie springs to (0,2).

Bessie walks from (0,2) to (1,2) (1 unit).

Bessie springs to (2,3).

Bessie walks from (2,3) to (3,3) (1 unit).

The total walking length of Bessie's path is 3 units.

Problem credits: Pedro Paredes

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