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-----
4 2
1 3
2 4

-----Sample Output-----
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.

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