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 Python (3.4.3) (judge verdict: Wrong Answer)

#!/Library/Frameworks/Python.framework/Versions/3.5/bin/python3

n = int(input())
a = list(map(int, input().split()))

ad = []
j = 1
for i in a:
    ad.append(i - j)
    j += 1

s = sum(ad)
avg = s / len(ad)

g1 = int(avg)
g2 = g1 - 1
g3 = g1 + 1
g4 = g1 - 2

d1 = 0
d2 = 0
d3 = 0
d4 = 0
j = 1
for i in a:
    d1 += abs(j + g1 - i)
    d2 += abs(j + g2 - i)
    d3 += abs(j + g3 - i)
    d4 += abs(j + g4 - i)
    j += 1
print(min(d1, d2, d3, d4))