#![allow(dead_code, unused_variables, unused_macros, unused_imports)]

fn recur(k: usize, tokyo: bool, d: i32, tk: &Vec<(i32, i32)>) -> i32 {
    if k == tk.len() {
        0
    } else {
        (if tokyo { tk[k].0 } else { tk[k].1 } + recur(k + 1, tokyo, d, tk))
            .max(if !tokyo { tk[k].0 } else { tk[k].1 } - d + recur(k + 1, !tokyo, d, tk))
    }
}

fn main() {
    input!(n:usize, d:i32,tk:[(i32,i32);n]);
    println!("{}", recur(0, true, d, &tk));
}

mod xxx {
    // https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
    #[macro_export]
    macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        input_inner!{$s, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}
    #[macro_export]
    macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};
    ($next:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

    #[macro_export]
    macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, bits)=>{
        u64::from_str_radix(&$next(), 2).expect("Parse error")
    };

    ($next:expr, hexs)=>{
        u64::from_str_radix(&$next(), 16).expect("Parse error")
    };


    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}
}