Problem:
Score : 300 points

Problem Statement

Snuke has an integer sequence A of length N.

He will freely choose an integer b.
Here, he will get sad if A_i and b+i are far from each other.
More specifically, the sadness of Snuke is calculated as follows:

abs(A_1 - (b+1)) + abs(A_2 - (b+2)) + ... + abs(A_N - (b+N))

Here, abs(x) is a function that returns the absolute value of x.

Find the minimum possible sadness of Snuke.

Constraints

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

1 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the minimum possible sadness of Snuke.

Sample Input 1

5
2 2 3 5 5

Sample Output 1

2

If we choose b=0, the sadness of Snuke would be abs(2-(0+1))+abs(2-(0+2))+abs(3-(0+3))+abs(5-(0+4))+abs(5-(0+5))=2.
Any choice of b does not make the sadness of Snuke less than 2, so the answer is 2.

Sample Input 2

9
1 2 3 4 5 6 7 8 9

Sample Output 2

0

Sample Input 3

6
6 5 4 3 2 1

Sample Output 3

18

Sample Input 4

7
1 1 1 1 2 3 4

Sample Output 4

6

Solution in Ruby (2.3.3) (judge verdict: Time Limit Exceeded)

n = gets.chomp.to_i
a = gets.chomp.split(" ").map(&:to_i)

def sum(b, a)
  s = 0
  a.each_with_index do |an, i|
    s += (an - (b + i + 1)).abs
  end
  s
end

def pp(x)
  if x.abs < 1 then
    return (x >= 0) ? 1 : -1
  end
  x
end

list = []
b = rand(a.min..a.max)
100000.times do
  bs = sum(b, a)
  list << bs
  bn, bp = sum(b-1, a), sum(b+1, a)
  p = (bn < bp ? -(bs - bn).abs : (bs - bp).abs) * 0.1
  b = b + pp(p)

  if list[-2] == (bn < bp ? bn : bp) then
    break
  end
end

puts list.min