/* 
 *     Author:  srtry
 *     Created: 2025-05-10T20:04:06+09:00
 *     Coding:  utf-8-unix
 */

use proconio::input;
use std::io::{stdout,Write,BufWriter};

fn main() {
    input!{
        n:usize, d:isize,
        mut tk:[[isize;2];n]
    }
    tk.insert(0,vec![0,0]);

    let out = stdout();
    let mut out = BufWriter::new(out.lock());
    
    let mut dp:Vec<Vec<isize>> = vec![vec![0,0];n+1];
    dp[1][0] = tk[1][0];
    dp[1][1] = tk[1][1] - d;
    
    for i in 2..=n {
        dp[i][0] = dp[i-1][0].max(dp[i-1][1]-d) + tk[i][0];
        dp[i][1] = dp[i-1][1].max(dp[i-1][0]-d) + tk[i][1];
    }
    write!(out, "{}", dp[n].iter().max().unwrap()).unwrap();
}