fn main() {
    proconio::input! {
        n: usize,
        d: i64,
        tk: [(i64, i64); n],
    }
    let mut dp = (0, i64::MIN);
    for (t, k) in tk.into_iter() {
        let tt = dp.0 + t;
        let tk = dp.0 + k - d;
        let kt = dp.1 + t - d;
        let kk = dp.1 + k;
        dp = (tt.max(kt), tk.max(kk));
        eprintln!("{} {}", dp.0, dp.1);
    }
    println!("{}", dp.0.max(dp.1));
}