//https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8 より
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

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

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

//

use std::cmp::*;

fn run() {
    input! {
        n: usize,
        k: u64,
        a: [u64; n],
        b: [u64; n],
    }
    assert!(1 <= n && n <= 100000);
    let p = 10u64.pow(8);
    assert!(k <= p);
    assert!(a.iter().all(|a| *a <= p));
    assert!(b.iter().all(|a| *a <= p));
    let mut g = vec![vec![]; n];
    for i in 0..(n - 1) {
        g[i].push((i + 1, a[i] + b[i + 1]));
        if i + 2 < n {
            g[i].push((i + 2, a[i] + b[i + 2] + k));
        }
    }
    let mut dp = vec![std::u64::MAX; n];
    dp[0] = 0;
    let mut h = std::collections::BinaryHeap::new();
    h.push(Reverse((0, 0)));
    while let Some(Reverse((d, v))) = h.pop() {
        if d > dp[v] {
            continue;
        }
        for &(u, w) in g[v].iter() {
            let d = d + w;
            if d < dp[u] {
                dp[u] = d;
                h.push(Reverse((d, u)));
            }
        }
    }
    let ans = dp.into_iter().max().unwrap();
    println!("{}", ans);
}

fn main() {
    run();
}