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 <math.h>
 
#define rep(i, n) for(int i = 0; i < n; i++)
#define in_arr(type, a, n) for(int i = 0; i < n; i++) scanf(type, &((a)[i]));
#define ll long long int
 
ll abs_ll(ll a);

int main(){
    int n;
    scanf("%d", &n);
    ll a[n];
    rep(i, n){
        scanf("%lld", &a[i]);
        a[i] -= i + 1;
    }

    double ave = 0;
    rep(i, n){
        ave += a[i];
    }
    ave /= n;
    ll b = ave < 0 ? (ll)(ave - 0.5) : (ll)(ave + 0.5) ;
    ll sum = 0;
    rep(i, n){
        sum += abs_ll(a[i] - b);
    }
    printf("%lld\n", sum);
    //printf("%lld", b);
    return 0;
}

ll abs_ll(ll a){
    return a < 0 ? -1 * a : a;
}