#[allow(unused_imports)]
use std::char::from_digit;
#[allow(unused_imports)]
use std::cmp::{max, min};
#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
#[allow(unused_imports)]
use std::fs::File;
#[allow(unused_imports)]
use std::io::prelude::*;
#[allow(unused_imports)]
use std::io::{stdin, stdout, BufReader, BufWriter, Stdin, Stdout, Write};
#[allow(unused_imports)]
use std::mem::swap;
#[allow(unused_imports)]
use std::str::FromStr;

type BoxResult<T> = Result<T, Box<std::error::Error>>;

fn main() {
    let mut scanner = Scanner::new(stdin());
    let params = scanner.read_vector::<i64>().unwrap();
    let (n, c) = (params[0] as usize, params[1]);
    let mut arr = vec![];
    for _ in 0..n {
        let line = scanner.read_vector::<i64>().unwrap();
        let (x, v) = (line[0], line[1]);
        arr.push((x, v));
    }
    let mut cumsum = vec![arr[0].1];
    for i in 0..(n - 1) {
        let prv = cumsum[i];
        cumsum.push(prv + arr[i + 1].1);
    }
    let mut left_maximum = vec![];
    let mut right_maximum = vec![];
    for i in 0..n {
        let prv_left = if i == 0 { 0 } else { left_maximum[i - 1] };
        let prv_right = if i == 0 { 0 } else { right_maximum[i - 1] };
        left_maximum.push(max(cumsum[i] - arr[i].0, prv_left));
        right_maximum.push(max(cumsum[i] - 2 * arr[i].0, prv_right));
    }
    let mut ans = left_maximum[n - 1];
    let mut sum = 0;
    for i in 1..(n + 1) {
        sum += arr[n - i].1;
        let left_sum = if i < n { sum - 2 * (c - arr[n - i].0) + left_maximum[n - i - 1] } else { 0 };
        let right_sum = if i < n { sum - (c - arr[n - i].0) + right_maximum[n - i - 1] } else { sum - (c - arr[n - i].0) };
        let best = max(left_sum, right_sum);
        ans = max(best, ans);
    }
    println!("{}", ans);
}


#[allow(dead_code)]
struct Scanner<R> {
    reader: BufReader<R>,
    buffer: String,
}
impl<R> Scanner<R>
where
    R: Read,
{
    #[allow(dead_code)]
    fn new(inner: R) -> Scanner<R> {
        let reader = BufReader::new(inner);
        let buffer = String::new();
        Scanner {
            reader: reader,
            buffer: buffer,
        }
    }
    #[allow(dead_code)]
    fn with_capacity(capacity: usize, inner: R) -> Scanner<R> {
        let reader = BufReader::with_capacity(capacity, inner);
        let buffer = String::new();
        Scanner {
            reader: reader,
            buffer: buffer,
        }
    }

    #[allow(dead_code)]
    fn read_line(&mut self) -> BoxResult<String> {
        self.reader.read_line(&mut self.buffer)?;
        let cloned = self.buffer.trim().to_string();
        self.buffer.clear();
        Ok(cloned)
    }

    #[allow(dead_code)]
    fn read<T>(&mut self) -> BoxResult<T>
    where
        T: FromStr,
        T::Err: std::fmt::Debug,
        T::Err: std::error::Error,
        T::Err: 'static,
    {
        self.reader.read_line(&mut self.buffer)?;
        let parsed = self.buffer.trim().parse()?;
        self.buffer.clear();
        Ok(parsed)
    }
    #[allow(dead_code)]
    fn read_vector<T>(&mut self) -> BoxResult<Vec<T>>
    where
        T: FromStr,
        T::Err: std::fmt::Debug,
        T::Err: std::error::Error,
        T::Err: 'static,
    {
        self.reader.read_line(&mut self.buffer)?;
        let parsed_vec = self
            .buffer
            .trim()
            .split_whitespace()
            .map(|s| s.parse())
            .collect::<Result<Vec<_>, _>>()?;
        self.buffer.clear();
        Ok(parsed_vec)
    }
    #[allow(dead_code)]
    fn read_matrix<T>(&mut self, lines: usize) -> BoxResult<Vec<Vec<T>>>
    where
        T: FromStr,
        T::Err: std::fmt::Debug,
        T::Err: std::error::Error,
        T::Err: 'static,
    {
        let mut parsed_matrix = vec![];
        for _ in 0..lines {
            self.reader.read_line(&mut self.buffer)?;
            parsed_matrix.push(
                self.buffer
                    .trim()
                    .split_whitespace()
                    .map(|s| s.parse())
                    .collect::<Result<Vec<_>, _>>()?,
            );
            self.buffer.clear();
        }
        Ok(parsed_matrix)
    }
    #[allow(dead_code)]
    fn read_chars(&mut self) -> BoxResult<Vec<char>> {
        self.reader.read_line(&mut self.buffer)?;
        let chars = self.buffer.trim().chars().collect::<Vec<_>>();
        self.buffer.clear();
        Ok(chars)
    }
}


Measured memory_bytes: 10492