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: Accepted)

#include<bits/stdc++.h>
#define INFTY 10000000000
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
typedef long long int ll;
const int MOD=1000000007;
const int di[4] = {-1,0,1,0};
const int dj[4] = {0,-1,0,1};
ll gcd(ll a, ll b) { return b?gcd(b,a%b):a;}


int main(){
    int n;cin>>n;
    vector<ll> a(n);

    rep(i,n){
        ll num;cin >> num;
        a[i] = num-(i+1);
    }
    sort(a.begin(),a.end());

    ll ans=0;
    rep(i,n){
        ans += abs(a[i]-a[n/2]);
    }
cout<<ans<<endl;
    return 0;
}