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 Rust (1.15.1) (judge verdict: Wrong Answer)

#![allow(unused_imports)]
#![cfg_attr(feature = "cargo-clippy", allow(redundant_field_names))]

use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
use std::io::{self, BufWriter, Read, Write};
use std::ops::{Add, Div, Mul, Sub};
use std::str::{self, FromStr};
use std::{cmp, fmt};

fn main() {
    let (n, a): (usize, Vec<i64>) = {
        let mut sc = InputScanOnce::new(io::stdin(), 1024);
        let n = sc.next();
        (n, sc.vec(n))
    };
    let n = n as i64;
    let a = a
        .into_iter()
        .enumerate()
        .map(|(i, x)| x - (i as i64) - 1)
        .collect::<Vec<_>>();
    let s = a.iter().cloned().sum::<i64>();
    let m = s % n;
    let b = if 2 * m > n {
        s / n + 1
    } else if s < 0 && -2 * m < n {
        s / n - 1
    } else {
        s / n
    };
    let r = a.into_iter().map(|x| (x - b).abs()).sum::<i64>();
    println!("{}", r);
}

struct InputScanOnce {
    buf: Vec<u8>,
    pos: usize,
}

#[allow(dead_code)]
impl InputScanOnce {
    fn new<R: Read>(mut reader: R, estimated: usize) -> Self {
        let mut buf = Vec::with_capacity(estimated);
        let _ = io::copy(&mut reader, &mut buf).unwrap();
        InputScanOnce { buf: buf, pos: 0 }
    }

    #[inline]
    fn next<T: FromStr>(&mut self) -> T
    where
        T::Err: fmt::Debug,
    {
        let mut start = None;
        loop {
            match (self.buf[self.pos], start.is_some()) {
                (b' ', true) | (b'\n', true) => break,
                (_, true) | (b' ', false) | (b'\n', false) => self.pos += 1,
                (_, false) => start = Some(self.pos),
            }
        }
        let target = &self.buf[start.unwrap()..self.pos];
        unsafe { str::from_utf8_unchecked(target) }.parse().unwrap()
    }

    fn vec<T: FromStr>(&mut self, n: usize) -> Vec<T>
    where
        T::Err: fmt::Debug,
    {
        (0..n).map(|_| self.next()).collect()
    }

    fn pairs<T1: FromStr, T2: FromStr>(&mut self, n: usize) -> Vec<(T1, T2)>
    where
        T1::Err: fmt::Debug,
        T2::Err: fmt::Debug,
    {
        (0..n).map(|_| (self.next(), self.next())).collect()
    }

    fn trios<T1: FromStr, T2: FromStr, T3: FromStr>(&mut self, n: usize) -> Vec<(T1, T2, T3)>
    where
        T1::Err: fmt::Debug,
        T2::Err: fmt::Debug,
        T3::Err: fmt::Debug,
    {
        (0..n)
            .map(|_| (self.next(), self.next(), self.next()))
            .collect()
    }

    fn mat<T: FromStr>(&mut self, m: usize, n: usize) -> Vec<Vec<T>>
    where
        T::Err: fmt::Debug,
    {
        (0..m).map(|_| self.vec(n)).collect()
    }

    fn strings_as_mat<T, F: FnMut(u8) -> T>(&mut self, h: usize, mut f: F) -> Vec<Vec<T>> {
        (0..h)
            .map(|_| {
                let l = self.next::<String>();
                l.as_bytes().iter().cloned().map(&mut f).collect()
            })
            .collect()
    }
}