use std::io::*;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let n: i64 = itr.next().unwrap().parse().unwrap();
    let a: Vec<i64> = (0..n)
        .map(|_| itr.next().unwrap().parse().unwrap())
        .collect();
    if n == 1 {
        println!("{}", a[0] - 1);
        return;
    }
    if n == 2 {
        println!("{}", a[0] + a[1] - 1);
        return;
    }

    let mut ans = 1 << 30;
    for i in 1..n - 1 {
        for j in 1..101 {
            let mut diff = a[i as usize] - j;
            let mut cnt = diff.abs();
            let mut k = 1;
            let mut leftok = false;
            let mut rightok = false;
            while i - k >= 0 {
                if j - k <= 0 {
                    cnt += a[(i - k) as usize];
                    diff += a[(i - k) as usize];
                } else {
                    cnt += (a[(i - k) as usize] - j + k).abs();
                    diff += a[(i - k) as usize] - j + k;
                }
                if j - k == 1 {
                    leftok = true;
                }
                k += 1;
            }
            k = 1;
            while i + k < n {
                if j - k <= 0 {
                    cnt += a[(i + k) as usize];
                    diff += a[(i + k) as usize];
                } else {
                    cnt += (a[(i + k) as usize] - j + k).abs();
                    diff += a[(i + k) as usize] - j + k;
                }
                if j - k == 1 {
                    rightok = true;
                }
                k += 1;
            }
            if leftok && rightok && diff >= 0 {
                ans = std::cmp::min(ans, diff + (cnt - diff) / 2);
            }
        }
    }
    println!("{}", ans);
}