#[doc = " https://github.com/hatoo/competitive-rust-snippets"]
#[allow(unused_imports)]
use std::cmp::{max, min, Ordering};
#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
#[allow(unused_imports)]
use std::io::{stdin, stdout, BufWriter, Write};
#[allow(unused_imports)]
use std::iter::FromIterator;
#[allow(unused_macros)]
macro_rules ! debug { ( $ ( $ a : expr ) ,* ) => { eprintln ! ( concat ! ( $ ( stringify ! ( $ a ) , " = {:?}, " ) ,* ) , $ ( $ a ) ,* ) ; } }
#[macro_export]
macro_rules ! input { ( source = $ s : expr , $ ( $ r : tt ) * ) => { let mut parser = Parser :: from_str ( $ s ) ; input_inner ! { parser , $ ( $ r ) * } } ; ( parser = $ parser : ident , $ ( $ r : tt ) * ) => { input_inner ! { $ parser , $ ( $ r ) * } } ; ( new_stdin_parser = $ parser : ident , $ ( $ r : tt ) * ) => { let stdin = std :: io :: stdin ( ) ; let reader = std :: io :: BufReader :: new ( stdin . lock ( ) ) ; let mut $ parser = Parser :: new ( reader ) ; input_inner ! { $ parser , $ ( $ r ) * } } ; ( $ ( $ r : tt ) * ) => { input ! { new_stdin_parser = parser , $ ( $ r ) * } } ; }
#[macro_export]
macro_rules ! input_inner { ( $ parser : ident ) => { } ; ( $ parser : ident , ) => { } ; ( $ parser : ident , $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let $ var = read_value ! ( $ parser , $ t ) ; input_inner ! { $ parser $ ( $ r ) * } } ; }
#[macro_export]
macro_rules ! read_value { ( $ parser : ident , ( $ ( $ t : tt ) ,* ) ) => { ( $ ( read_value ! ( $ parser , $ t ) ) ,* ) } ; ( $ parser : ident , [ $ t : tt ; $ len : expr ] ) => { ( 0 ..$ len ) . map ( | _ | read_value ! ( $ parser , $ t ) ) . collect ::< Vec < _ >> ( ) } ; ( $ parser : ident , chars ) => { read_value ! ( $ parser , String ) . chars ( ) . collect ::< Vec < char >> ( ) } ; ( $ parser : ident , usize1 ) => { read_value ! ( $ parser , usize ) - 1 } ; ( $ parser : ident , $ t : ty ) => { $ parser . next ::<$ t > ( ) . expect ( "Parse error" ) } ; }
use std::io;
use std::io::BufRead;
use std::str;
pub struct Parser<R> {
    reader: R,
    buf: Vec<u8>,
    pos: usize,
}
impl Parser<io::Empty> {
    pub fn from_str(s: &str) -> Parser<io::Empty> {
        Parser {
            reader: io::empty(),
            buf: s.as_bytes().to_vec(),
            pos: 0,
        }
    }
}
impl<R: BufRead> Parser<R> {
    pub fn new(reader: R) -> Parser<R> {
        Parser {
            reader: reader,
            buf: vec![],
            pos: 0,
        }
    }
    pub fn update_buf(&mut self) {
        self.buf.clear();
        self.pos = 0;
        loop {
            let (len, complete) = {
                let buf2 = self.reader.fill_buf().unwrap();
                self.buf.extend_from_slice(buf2);
                let len = buf2.len();
                if len == 0 {
                    break;
                }
                (len, buf2[len - 1] <= 0x20)
            };
            self.reader.consume(len);
            if complete {
                break;
            }
        }
    }
    pub fn next<T: str::FromStr>(&mut self) -> Result<T, T::Err> {
        loop {
            let mut begin = self.pos;
            while begin < self.buf.len() && (self.buf[begin] <= 0x20) {
                begin += 1;
            }
            let mut end = begin;
            while end < self.buf.len() && (self.buf[end] > 0x20) {
                end += 1;
            }
            if begin != self.buf.len() {
                self.pos = end;
                return str::from_utf8(&self.buf[begin..end]).unwrap().parse::<T>();
            } else {
                self.update_buf();
            }
        }
    }
}
#[allow(unused_macros)]
macro_rules ! debug { ( $ ( $ a : expr ) ,* ) => { eprintln ! ( concat ! ( $ ( stringify ! ( $ a ) , " = {:?}, " ) ,* ) , $ ( $ a ) ,* ) ; } }
#[doc = " https://github.com/hatoo/competitive-rust-snippets"]
const BIG_STACK_SIZE: bool = true;
#[allow(dead_code)]
fn main() {
    use std::thread;
    if BIG_STACK_SIZE {
        thread::Builder::new()
            .stack_size(32 * 1024 * 1024)
            .name("solve".into())
            .spawn(solve)
            .unwrap()
            .join()
            .unwrap();
    } else {
        solve();
    }
}
fn solve() {
    input!{
        N:usize,C:i64,
        xv: [(i64,i64);N],
    }
    let mut xv_rev=vec![];
    for i in (0..N).rev() {
        let (x,v)=xv[i];
        xv_rev.push((C-x,v));
    }

    let mut tmp=vec![0;N];
    let mut tmp_rev=vec![0;N];
    for i in 0..N {
        let (x,v)=xv[i];
        tmp[i]=v;
    }
    for i in 0..N {
        let (x,v)=xv_rev[i];
        tmp_rev[i]=v;
    }
    let cumsumr=cumsum1(&tmp);
    let cumsuml=cumsum1(&tmp_rev);

    let mut segr: SEG<MAX> = SEG::new(N);
    let mut segl: SEG<MAX> = SEG::new(N);
    for i in 0..N {
        let (x,_)=xv[i];
        segr.update(i, cumsumr[i+1]-x);
    }
    for i in 0..N {
        let (x,_)=xv_rev[i];
        segl.update(i, cumsuml[i+1]-x);
    }

    let all_r=segr.query(0, N);
    let all_l=segl.query(0, N);

    let mut partial_r=0;
    for r in 0..N {
        let mut v=cumsumr[r+1];
        v-=2*xv[r].0;
        v+=segl.query(0, N-r-1);
        partial_r=max(partial_r,v);
    }

    let mut partial_l=0;
    for l in 0..N {
        let mut v=cumsuml[l+1];
        v-=2*xv_rev[l].0;
        v+=segr.query(0, N-l-1);
        partial_l=max(partial_l,v);
    }
    let vs = vec![all_r, all_l, partial_r, partial_l];
    println!("{}",vec_max(&vs));
}
fn vec_max<T: Ord + Clone>(xs: &[T]) -> T {
    let mut v = &xs[0];
    for x in xs {
        if x > v {
            v = x;
        }
    }
    v.clone()
}
fn cumsum1(init: &[i64]) -> Vec<i64> {
    let n = init.len();
    let mut dp = vec![0; n + 1];
    let mut acc = 0;
    for i in 0..n {
        acc += init[i];
        dp[i + 1] = acc;
    }
    dp
}

#[allow(dead_code)]
pub trait Monoid {
    type T: Clone;
    fn id() -> Self::T;
    fn op(a: &Self::T, b: &Self::T) -> Self::T;
}
#[allow(dead_code)]
pub struct SEG<M: Monoid> {
    n: usize,
    buf: Vec<M::T>,
}
impl<M: Monoid> SEG<M> {
    #[allow(dead_code)]
    pub fn new(n: usize) -> SEG<M> {
        let mut m = 1;
        while m < n {
            m *= 2;
        }
        SEG {
            n: m,
            buf: vec![M::id().clone(); 2 * m - 1],
        }
    }
    #[allow(dead_code)]
    pub fn update(&mut self, k: usize, a: M::T) {
        let mut k = k + self.n - 1;
        self.buf[k] = a;
        while k > 0 {
            k = (k - 1) / 2;
            self.buf[k] = M::op(&self.buf[k * 2 + 1], &self.buf[k * 2 + 2]);
        }
    }
    #[allow(dead_code)]
    pub fn get(&self, k: usize) -> M::T {
        self.buf[k + self.n].clone()
    }
    pub fn do_query(&self, a: usize, b: usize, k: usize, l: usize, r: usize) -> M::T {
        if r <= a || b <= l {
            return M::id();
        }
        if a <= l && r <= b {
            return self.buf[k].clone();
        } else {
            let vl = self.do_query(a, b, k * 2 + 1, l, (l + r) / 2);
            let vr = self.do_query(a, b, k * 2 + 2, (l + r) / 2, r);
            return M::op(&vl, &vr);
        }
    }
    #[allow(dead_code)]
    pub fn query(&self, a: usize, b: usize) -> M::T {
        self.do_query(a, b, 0, 0, self.n)
    }
}
struct MAX;
impl Monoid for MAX {
    type T = i64;
    fn id() -> Self::T {
        0
    }
    fn op(a: &Self::T, b: &Self::T) -> Self::T {
        std::cmp::max(*a, *b)
    }
}

Measured memory_bytes: 22908