use std::cmp;
fn main() {
    let mut sc = Scanner::new();
    let n: usize = sc.read();
    let c: i64 = sc.read();
    let mut xs = Vec::new();
    let mut vs = Vec::new();
    for _ in 0..n {
        let x: i64 = sc.read();
        let v: i64 = sc.read();
        xs.push(x);
        vs.push(v);
    }

    let mut right = vec![0; n];
    for i in 0..n {
        if i == 0 {
            right[i] = vs[i] - xs[i];
        } else {
            right[i] = right[i - 1] + vs[i] - (xs[i] - xs[i - 1]);
        }
    }

    let mut left = vec![0; n];
    for i in (0..n).rev() {
        if i == n - 1 {
            left[i] = vs[i] - (c - xs[i]);
        } else {
            left[i] = left[i + 1] + vs[i] - (xs[i + 1] - xs[i]);
        }
    }

    let mut right_max = vec![0; n];
    let mut left_max = vec![0; n];
    for i in 0..n {
        if i == 0 {
            right_max[i] = right[i];
        } else {
            right_max[i] = cmp::max(right[i], right_max[i - 1]);
        }
    }
    for i in (0..n).rev() {
        if i == n - 1 {
            left_max[i] = left[i];
        } else {
            left_max[i] = cmp::max(left[i], left_max[i + 1]);
        }
    }

    let mut max = 0;
    for i in 0..n {
        let ans = if i == n - 1 {
            right_max[n - 1]
        } else {
            cmp::max(
                right_max[i] + cmp::max(left_max[i + 1] - xs[i], 0),
                cmp::max(right_max[i] - (c - xs[i + 1]), 0) + left_max[i + 1],
            )
        };
        max = cmp::max(max, ans);
    }
    max = cmp::max(max, left_max[0]);
    println!("{}", max);
}

struct Scanner {
    ptr: usize,
    length: usize,
    buf: Vec<u8>,
    small_cache: Vec<u8>,
}

impl Scanner {
    fn new() -> Scanner {
        Scanner {
            ptr: 0,
            length: 0,
            buf: vec![0; 1024],
            small_cache: vec![0; 1024],
        }
    }

    fn load(&mut self) {
        use std::io::Read;
        let mut s = std::io::stdin();
        self.length = s.read(&mut self.buf).unwrap();
    }

    fn byte(&mut self) -> u8 {
        if self.ptr >= self.length {
            self.ptr = 0;
            self.load();
            if self.length == 0 {
                self.buf[0] = b'\n';
                self.length = 1;
            }
        }

        self.ptr += 1;
        return self.buf[self.ptr - 1];
    }

    fn is_space(b: u8) -> bool {
        b == b'\n' || b == b'\r' || b == b'\t' || b == b' '
    }

    fn read<T>(&mut self) -> T
    where
        T: std::str::FromStr,
        T::Err: std::fmt::Debug,
    {
        let mut b = self.byte();
        while Scanner::is_space(b) {
            b = self.byte();
        }

        for pos in 0..self.small_cache.len() {
            self.small_cache[pos] = b;
            b = self.byte();
            if Scanner::is_space(b) {
                return String::from_utf8_lossy(&self.small_cache[0..(pos + 1)])
                    .parse()
                    .unwrap();
            }
        }

        let mut v = self.small_cache.clone();
        while !Scanner::is_space(b) {
            v.push(b);
            b = self.byte();
        }
        return String::from_utf8_lossy(&v).parse().unwrap();
    }
}


Measured memory_bytes: 10492