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?

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.

Constraints

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

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

见题面 Input 部分

见题面 Output 部分