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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>
#include<stdlib.h>

#define min(a,b) ((a)<(b)?(a):(b))
#define INF 200000000

int comp(const void *a, const void *b)
{
    return *(long*)a - *(long*)b;
}

int main(){
	int n,i;
	long a[200000],ave,ave2,ans=0,ans2=INF;
	scanf("%d",&n);
	for(i=0;i<n;i++){
		scanf("%ld",&a[i]);
		a[i]-=i;
	}
	qsort(a,n,sizeof(long),comp);
	ave=a[n/2+1];
	if(n%2==0){
		ave2=a[n/2];
		ans2=0;
	}
	for(i=0;i<n;i++){
		ans+=abs(ave-a[i]);
		if(n%2==0)ans2=abs(ave2-a[i]);
	}
	ans=min(ans,ans2);
	printf("%ld\n",ans);
	return 0;
}