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: Wrong Answer)

n=gets.to_i
a=gets.split(' ').map(&:to_i)
a.map!.with_index{|e,i|e-i-1}
k = ((a.inject(0.0){|r,i| r+=i })/n).round
min = a.inject(0.0){|r,i| r+=(i-k).abs }.to_i
ai = a.min
aa = a.max
while 1
  al = ((ai+k)/2).round
  ah = ((aa+k)/2).round
  m1 = a.inject(0.0){|r,i| r+=(i-al).abs }.to_i
  m2 = a.inject(0.0){|r,i| r+=(i-ah).abs }.to_i
  if min>m1
    min = m1
    ai = al
  elsif min>m2
    min = m2
    aa = ah
  else
      break
  end
end
p min