use std::io::*;
use std::str::FromStr;

fn main() {
    let n: usize = read();
    let d: isize = read();
    let mut t = vec![0isize; n];
    let mut k = vec![0isize; n];
    for _i in 0..n{
        t[_i] = read();
        k[_i] = read();
    }
    let mut t_dp = vec![0isize; n];
    let mut k_dp = vec![0isize; n];
    t_dp[0] = t[0];
    k_dp[0] = k[0] - d;
    for _i in 0..n - 1{
        t_dp[_i + 1] = (t_dp[_i]).max(k_dp[_i] - d) + t[_i + 1];
        k_dp[_i + 1] = k_dp[_i].max(t_dp[_i] - d) + k[_i + 1];
    }
    println!("{}",t_dp[n - 1].max(k_dp[n - 1]));
}

pub fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}