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C - Prison

Score : $300$ points

Problem Statement

We have $N$ ID cards, and there are $M$ gates.

We can pass the $i$-th gate if we have one of the following ID cards: the $L_i$-th, $(L_i+1)$-th, ..., and $R_i$-th ID cards.

How many of the ID cards allow us to pass all the gates alone?

Constraints

• All values in input are integers.
• $1 \leq N \leq 10^5$
• $1 \leq M \leq 10^5$
• $1 \leq L_i \leq R_i \leq N$

Input

Input is given from Standard Input in the following format:

$N$ $M$

$L_1$ $R_1$

$L_2$ $R_2$

$\vdots$

$L_M$ $R_M$

Output

Print the number of ID cards that allow us to pass all the gates alone.

4 2
1 3
2 4

2

Two ID cards allow us to pass all the gates alone, as follows:

• The first ID card does not allow us to pass the second gate.
• The second ID card allows us to pass all the gates.
• The third ID card allows us to pass all the gates.
• The fourth ID card does not allow us to pass the first gate.

10 3
3 6
5 7
6 9

1

100000 1
1 100000

100000