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.Linq;
using static System.Console;

namespace practice01
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(ReadLine());
            int[] a = ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
            int[] b = new int[n];
            for(int i = 1; i <= n; i++)
            {
                b[i - 1] = a[i-1]-i;
            }
            int ave1 = (int)Math.Floor(b.Average());
            int ave2 = (int)Math.Ceiling(b.Average());
            int result1 = 0;
            int result2 = 0;
            for (int i = 1; i <= n; i++)
            {
                result1 += Math.Abs(a[i-1] - (ave1+i));
            }
            for (int i = 1; i <= n; i++)
            {
                result2 += Math.Abs(a[i - 1] - (ave2 + i));
            }

            WriteLine(result1 < result2 ? result1: result2);
        }
    }
}