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.