Problem:
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

Solution in Rust (1.15.1) (judge verdict: Accepted)

// This code is generated by [cargo-atcoder](https://github.com/tanakh/cargo-atcoder)
// Original source code:
/*
#[allow(unused_macros)]
macro_rules! debug {
	($($a:expr),*) => {
		println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
	}
}

#[allow(dead_code)]
const MOD: usize = 1e9 as usize + 7;

fn main() {
	let (r, w) = (std::io::stdin(), std::io::stdout());
	let mut sc = IO::new(r.lock(), w.lock());

	let n: usize = sc.read();
	let m: usize = sc.read();
	let lr: Vec<(i32, i32)> = sc.read_pairs(m);

	let mut l_max = std::i32::MIN;
	let mut r_min = std::i32::MAX;
	for (l, r) in &lr {
		l_max = std::cmp::max(l_max, *l);
		r_min = std::cmp::min(r_min, *r);
	}
	println!("{}", std::cmp::max(0, r_min - l_max + 1));
}

pub struct IO<R, W: std::io::Write>(R, std::io::BufWriter<W>);

impl<R: std::io::Read, W: std::io::Write> IO<R, W> {
	pub fn new(r: R, w: W) -> IO<R, W> {
		IO(r, std::io::BufWriter::new(w))
	}
	pub fn write<S: std::ops::Deref<Target = str>>(&mut self, s: S) {
		use std::io::Write;
		self.1.write(s.as_bytes()).unwrap();
	}
	pub fn read<T: std::str::FromStr>(&mut self) -> T {
		use std::io::Read;
		let buf = self
			.0
			.by_ref()
			.bytes()
			.map(|b| b.unwrap())
			.skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r' || b == b'\t')
			.take_while(|&b| b != b' ' && b != b'\n' && b != b'\r' && b != b'\t')
			.collect::<Vec<_>>();
		unsafe { std::str::from_utf8_unchecked(&buf) }
			.parse()
			.ok()
			.expect("Parse error.")
	}
	pub fn read_vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
		(0..n).map(|_| self.read()).collect()
	}
	pub fn read_chars(&mut self) -> Vec<char> {
		self.read::<String>().chars().collect()
	}
	pub fn read_pairs<T: std::str::FromStr>(&mut self, n: usize) -> Vec<(T, T)> {
		(0..n)
			.map(|_| (self.read::<T>(), self.read::<T>()))
			.collect::<Vec<(T, T)>>()
	}
	pub fn read_matrix<T: std::str::FromStr>(&mut self, n: usize, m: usize) -> Vec<Vec<T>> {
		(0..n)
			.map(|_| (0..m).map(|_| self.read()).collect())
			.collect()
	}
}
*/

fn main() {
    let exe = "/tmp/binF86CCB21";
    std::io::Write::write_all(&mut std::fs::File::create(exe).unwrap(), &decode(BIN)).unwrap();
    std::fs::set_permissions(exe, std::os::unix::fs::PermissionsExt::from_mode(0o755)).unwrap();
    std::process::exit(std::process::Command::new(exe).status().unwrap().code().unwrap())
}

fn decode(v: &str) -> Vec<u8> {
    let mut ret = vec![];
    let mut buf = 0;
    let mut tbl = vec![64; 256];
    for i in 0..64 { tbl[TBL[i] as usize] = i as u8; }
    for (i, c) in v.bytes().filter_map(|c| { let c = tbl[c as usize]; if c < 64 { Some(c) } else { None } }).enumerate() {
        match i % 4 {
            0 => buf = c << 2,
            1 => { ret.push(buf | c >> 4); buf = c << 4; }
            2 => { ret.push(buf | c >> 2); buf = c << 6; }
            3 => ret.push(buf | c),
            _ => unreachable!(),
        }
    }
    ret
}

const TBL: &'static [u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
const BIN: &'static str = "<base64-encoded ELF executable removed>";