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: usize = itr.next().unwrap().parse().unwrap();
    let mut a: Vec<usize> = (0..n)
        .map(|_| itr.next().unwrap().parse().unwrap())
        .collect();

    let sum = a.iter().fold(0, |acc, x| acc + x);
    let mut height = 1;
    while height * (height + 1) + height + 1 <= sum {
        height += 1;
    }

    let hidx = height - 1;
    a.resize(hidx * 2 + 1, 0);
    let excess = sum - height * (height - 1) - height;
    let mut need = 0;
    if a[hidx] < height {
        need = height - a[hidx];
    }
    for i in 0..hidx {
        if i < hidx * 2 + 1 && a[i] < i + 1 {
            need += i + 1 - a[i];
        }

        if hidx * 2 - i < hidx * 2 + 1 && a[hidx * 2 - i] < i + 1 {
            need += i + 1 - a[hidx * 2 - i];
        }
    }
    println!("{}", excess + need);
}