Problem:
Score : 600 points

Problem Statement

There are N cities in Republic of AtCoder. The size of the i-th city is A_{i}.
Takahashi would like to build N-1 bidirectional roads connecting two cities so that any city can be reached from any other city by using these roads.

Assume that the cost of building a road connecting the i-th city and the j-th city is |i-j| \times D + A_{i} + A_{j}.
For Takahashi, find the minimum possible total cost to achieve the objective.

Constraints

1 \leq N \leq 2 \times 10^5

1 \leq D \leq 10^9

1 \leq A_{i} \leq 10^9

A_{i} and D are integers.

Input

Input is given from Standard Input in the following format:

N D
A_1 A_2 ... A_N

Output

Print the minimum possible total cost.

Sample Input 1

3 1
1 100 1

Sample Output 1

106

This cost can be achieved by, for example, building roads connecting City 1, 2 and City 1, 3.

Sample Input 2

3 1000
1 100 1

Sample Output 2

2202

Sample Input 3

6 14
25 171 7 1 17 162

Sample Output 3

497

Sample Input 4

12 5
43 94 27 3 69 99 56 25 8 15 46 8

Sample Output 4

658

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::cmp;

const INF: usize = 1e15 as usize;

fn main() {
    let s = std::io::stdin();
    let mut sc = Scanner { reader: s.lock() };
    let n: usize = sc.read();
    let d: usize = sc.read();
    let a: Vec<usize> = sc.read_vec(n);
    let mut ai: Vec<(usize, usize)> = a.iter().enumerate().map(|(i, &a)| (a, i)).collect();
    ai.sort();


    let mut edges = vec![];
    let mut seg_left = SegmentTree::new(n, (INF, 0), |a, b| cmp::min(a, b));
    let mut seg_right = SegmentTree::new(n, (INF, 0), |a, b| cmp::min(a, b));
    for &(_, i) in ai.iter() {
        let (left_cost, left_i) = seg_left.query(0, i + 1);
        if left_cost != INF {
            let cost = (i - left_i) * d + a[i] + a[left_i];
            edges.push((cost, i, left_i));
        }
        let (right_cost, right_i) = seg_right.query(i, n);
        if right_cost != INF {
            let cost = (right_i - i) * d + a[i] + a[right_i];
            edges.push((cost, i, right_i));
        }
        seg_left.update(i, ((n - i) * d + a[i], i));
        seg_right.update(i, (i * d + a[i], i));
    }

    edges.sort();
    let mut ans = 0;
    let mut uf = UnionFind::new(n);
    for &(cost, from, to) in edges.iter() {
        if uf.find(from) != uf.find(to) {
            uf.unite(from, to);
            ans += cost;
        }
    }

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

pub struct UnionFind {
    parent: Vec<usize>,
    sizes: Vec<usize>,
    size: usize,
}

impl UnionFind {
    pub fn new(n: usize) -> UnionFind {
        UnionFind {
            parent: (0..n).map(|i| i).collect::<Vec<usize>>(),
            sizes: vec![1; n],
            size: n,
        }
    }

    pub fn find(&mut self, x: usize) -> usize {
        if x == self.parent[x] {
            x
        } else {
            let px = self.parent[x];
            self.parent[x] = self.find(px);
            self.parent[x]
        }
    }

    pub fn unite(&mut self, x: usize, y: usize) -> bool {
        let parent_x = self.find(x);
        let parent_y = self.find(y);
        if parent_x == parent_y {
            return false;
        }

        let (large, small) = if self.sizes[parent_x] < self.sizes[parent_y] {
            (parent_y, parent_x)
        } else {
            (parent_x, parent_y)
        };

        self.parent[small] = large;
        self.sizes[large] += self.sizes[small];
        self.sizes[small] = 0;
        self.size -= 1;
        return true;
    }
}

/// Segment Tree for range queries
pub struct SegmentTree<T, F> {
    seg: Vec<T>,
    n: usize,
    f: F,
    initial_value: T,
}

impl<T: Copy, F> SegmentTree<T, F>
    where
        F: Fn(T, T) -> T,
{
    pub fn new(size: usize, initial_value: T, f: F) -> SegmentTree<T, F> {
        let mut m = 1;
        while m <= size {
            m <<= 1;
        }
        SegmentTree {
            seg: vec![initial_value; m * 2],
            n: m,
            f: f,
            initial_value: initial_value,
        }
    }

    pub fn update(&mut self, k: usize, value: T) {
        let mut k = k;
        k += self.n - 1;
        self.seg[k] = value;
        while k > 0 {
            k = (k - 1) >> 1;
            self.seg[k] = (self.f)(self.seg[k * 2 + 1], self.seg[k * 2 + 2]);
        }
    }

    /// Get the minimum value in the array in the range [a, b)
    ///
    /// # Panics
    ///
    /// Panics if `a >= b`.
    pub fn query(&self, a: usize, b: usize) -> T {
        assert!(a < b);
        self.query_range(a, b, 0, 0, self.n)
    }

    fn query_range(&self, a: usize, b: usize, k: usize, l: usize, r: usize) -> T {
        if r <= a || b <= l {
            self.initial_value
        } else if a <= l && r <= b {
            self.seg[k]
        } else {
            let x = self.query_range(a, b, k * 2 + 1, l, (l + r) >> 1);
            let y = self.query_range(a, b, k * 2 + 2, (l + r) >> 1, r);
            (self.f)(x, y)
        }
    }
}

pub struct Scanner<R> {
    reader: R,
}

impl<R: std::io::Read> Scanner<R> {
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self
            .reader
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n')
            .take_while(|&b| b != b' ' && b != b'\n')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn read_vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }
    pub fn chars(&mut self) -> Vec<char> {
        self.read::<String>().chars().collect()
    }
}