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Contest: Task: Related: TaskB TaskD

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.

Sample Input 1

4 2
1 3
2 4

Sample Output 1

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.

Sample Input 2

10 3
3 6
5 7
6 9

Sample Output 2

1

Sample Input 3

100000 1
1 100000

Sample Output 3

100000