#![allow(unused_imports)]
#![allow(non_snake_case)]
use std::collections::HashMap;

#[allow(unused_macros)]
macro_rules! read {
    ([$t:ty] ; $n:expr) =>
        ((0..$n).map(|_| read!([$t])).collect::<Vec<_>>());
    ($($t:ty),+ ; $n:expr) =>
        ((0..$n).map(|_| read!($($t),+)).collect::<Vec<_>>());
    ([$t:ty]) =>
        (rl().split_whitespace().map(|w| w.parse().unwrap()).collect::<Vec<$t>>());
    ($t:ty) =>
        (rl().parse::<$t>().unwrap());
    ($($t:ty),*) => {{
        let buf = rl();
        let mut w = buf.split_whitespace();
        ($(w.next().unwrap().parse::<$t>().unwrap()),*)
    }};
}

#[allow(dead_code)]
fn rl() -> String {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    buf.trim_end().to_owned()
}

trait IteratorExt: Iterator + Sized {
    fn vec(self) -> Vec<Self::Item> {
        self.collect()
    }
}
impl<T: Iterator> IteratorExt for T {}

fn main() {
    let (n, d) = read!(usize, i64);
    let v = read!(i64, i64; n);

    let ans = {
        let mut tmp = (v[n-1].0, v[n-1].1);
        if n > 1 {
            for i in (0..=n-2).rev() {
                tmp = (
                    (v[i].0 + tmp.0).max(v[i].0 + tmp.1 - d),
                    (v[i].1 + tmp.1).max(v[i].1 + tmp.0 - d)
                );
            }    
        }
        tmp
    };

    println!("{}", ans.0.max(ans.1 - d));
}