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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <algorithm>
#include <cmath>
#include <climits>

using namespace std;

int main() {
  int n;
  cin >> n;

  long a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }

  long min_a = INT_MAX;
  for (int i = 0; i < n; i++) {
    a[i] -= (i + 1);
    min_a = min(min_a, a[i]);
  }

  // aは全て正の整数
  if (min_a < 0) {
    for (int i = 0; i < n; i++) {
      a[i] += abs(0 - min_a);
    }
  }

  long sum = 0;
  for (int i = 0; i < n; i++) {
    sum += a[i];
  }

  long ave = sum / n;

  long ans = 0;
  for (int i = 0; i < n; i++) {
    ans += abs(ave - a[i]);
  }

  long ans1 = 0;
  ave += 1;
  for (int i = 0; i < n; i++) {
    ans1 += abs(ave - a[i]);
  }
  ans = min(ans, ans1);

  long ans2 = 0;
  ave -= 2;
  for (int i = 0; i < n; i++) {
    ans2 += abs(ave - a[i]);
  }
  ans = min(ans, ans2);

  cout << ans << endl;
}