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 Python (3.4.3) (judge verdict: Wrong Answer)

# E
N, D = map(int, input().split())
A_list = list(map(int, input().split()))

# minimum spanning tree

res = 0

# Prim based
B_list = [0]*N
for i in range(N):
    B_list[i] = A_list[i] + D*i
    
C_list = [0]*N
for i in range(N):
    C_list[i] = A_list[i] + D*(N-i)
    
# cummin seen from left
B_cummmin = [0]*N
R = B_list[N-1]
BA = N-1
for i in range(N-1, -1, -1):
    if B_list[i] <= R:
        R = B_list[i]
        BA = i
    B_cummmin[i] = BA

# cummin seen from right
C_cummmin = [0]*N
R = C_list[0]
CA = 0
for i in range(N):
    if C_list[i] <= R:
        R = C_list[i]
        CA = i
    C_cummmin[i] = CA

# start from 0
start = 0
while start < N-1:
    end = B_cummmin[start+1]
    target = C_cummmin[end-1]
    if target > start:
        target = start
    res += D*(end - target) + A_list[target] + A_list[end]
    for i in range(start+1, end):
        ds = D*(i - target) + A_list[target] + A_list[i]
        de = D*(end - i) + A_list[end] + A_list[i]
        res += min(ds, de)
    start = end

print(res)