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 <bits/stdc++.h>
using namespace std;

int main(){

    long long int N,sum=0,ans_1=0,ans_2=0;
    cin>>N;
    vector<long long int> S(N);
    for(int i=0;i<N;i++){
        cin>>S.at(i);
        sum=sum+S.at(i)-i-1;
    }
    if(sum>=0){
        for(int i=0;i<N;i++){
            ans_1=ans_1+abs(S.at(i)-i-sum/N-1);
            ans_2=ans_2+abs(S.at(i)-i-sum/N-2);
        }
        cout<<min(ans_1,ans_2)<<endl;
        }
    else{
        /*for(int i=0;i<N;i++){
                ans_1=ans_1+abs(S.at(i)-i-sum/N-1);
                ans_2=ans_2+abs(S.at(i)-i-sum/N);
            }
        cout<<min(ans_1,ans_2)<<endl;*/
        cout<<-5<<endl;
        }
    }