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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Math;

class Program{
    static void Main(){
        int med = 0;
        int sorrow = 0;
        int N = int.Parse(Console.ReadLine());
        string[] S = Console.ReadLine().Split(' ');
        int[] A = new int[N];
        int[] B = new int[N];
        for(int i = 0; i < N; i++){
            A[i] = int.Parse(S[i]);
            B[i] = A[i] - (i+1);
        }

        Array.Sort(B);
        if(N % 2 == 1){
            med = B[N/2];
        }
        else{
            med = (B[N/2-1] + B[N/2])/2;
        }

        for(int j = 0; j < N; j++){
            sorrow += Abs(B[j] - med);
        }
        Console.WriteLine(sorrow);
    }
}