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use std::cmp::max;

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>>()
    };

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

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

fn get_amount_of_gravel(height: i64, target: i64, k: i64) -> i64 {
    max(target - k - height, 0)
}

fn main() {
    input! {
        n: usize,
        k: i64,
        a: [i64; n],
    }

    let mut a = a;
    let mut result = 0;

    if n == 1 {
        println!("0");
        return;
    }


    // eprintln!("{:?}", a);

    for i in 1..n {
        let required_gravel = get_amount_of_gravel(a[i], a[i - 1], k);

        result += required_gravel;
        a[i] += required_gravel;
    }

    // eprintln!("{:?}", a);

    for i in (0..=(n - 2)).rev() {
        let required_gravel = get_amount_of_gravel(a[i], a[i + 1], k);

        // eprintln!("{} {}", a[i], a[i+1]);

        result += required_gravel;
        a[i] += required_gravel;
    }

    // eprintln!("{:?}", a);

    println!("{}", result);
}