fn main() {
    let mut input = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut input).ok();
    let input: Vec<i64> = input
        .split_whitespace()
        .map(|n| n.parse().unwrap())
        .collect();

    let d = input[1];
    let mut income = (0, -d);

    for tk in input[2..].chunks(2) {
        income = (
            income.0.max(income.1 - d) + tk[0],
            income.1.max(income.0 - d) + tk[1],
        );
    }

    println!("{}", income.0.max(income.1));
}