#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::{Write, BufWriter};
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    ($($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_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

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

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

fn solve() {
    let out = std::io::stdout();
    let mut out = BufWriter::new(out.lock());
    macro_rules! puts {
        ($($format:tt)*) => (write!(out,$($format)*).unwrap());
    }
    input! {
        n: usize,
        l: i64,
        x: [i64; n],
        y: [i64; n],
    }
    let mut st = vec![];
    for i in 0..n {
        st.push((x[i], 1));
        st.push((y[i], -1));
    }
    st.sort();
    let mut acc = vec![0; 2 * n + 1];
    for i in 0..2 * n {
        acc[i + 1] = acc[i] + st[i].1;
    }
    let mut mi = (0, 0);
    for i in 0..2 * n {
        mi = min(mi, (acc[i], i));
    }
    let mut ma = (0, 0);
    for i in 0..2 * n {
        let diff = if i < mi.1 { if acc[i] > 0 { -1 } else { 0 } } else { 0 };
        ma = max(ma, (acc[i] - mi.0 + diff, (i + 2 * n - mi.1) % (2 * n)));
    }
    eprintln!("mi = {:?}, ma = {:?}", mi, ma);
    let mut tot = l * (ma.0 - 1) + st[(ma.1 + mi.1) % (2 * n)].0;
    if ma.1 + mi.1 >= 2 * n {
        tot += l;
    }
    puts!("{}\n", tot);
}

fn main() {
    // In order to avoid potential stack overflow, spawn a new thread.
    let stack_size = 104_857_600; // 100 MB
    let thd = std::thread::Builder::new().stack_size(stack_size);
    thd.spawn(|| solve()).unwrap().join().unwrap();
}