use std::io::Read;
use std::collections::VecDeque;

fn main() {
    let mut all_data = String::new();
    std::io::stdin().read_to_string(&mut all_data).ok();
    let all_data: Vec<&str> = all_data.trim().split('\n').map(|s| s.trim()).collect();
    let n: usize = all_data.iter().next().unwrap().parse::<usize>().unwrap();
    let b: Vec<usize> = all_data.iter().skip(1).next().unwrap().split_whitespace().map(|s| s.parse::<usize>().unwrap()).collect();

    let mut empty_indices = VecDeque::new();
    let mut overed_indices = VecDeque::new();
    let mut cost: usize = 0;

    b.iter().enumerate().for_each(|pair| {
        let index = pair.0;
        let count = pair.1;
        if count == &0 {
            if overed_indices.is_empty() {
                empty_indices.push_back(index);
            } else {
                cost += index - overed_indices.pop_back().unwrap();
            }
        } else if count > &1 {
            if empty_indices.is_empty() {
                (0..(count - 1)).for_each(|_| {
                    overed_indices.push_back(index);
                });
            } else {
                let mut iter_count = count - 1;
                while !empty_indices.is_empty() && iter_count > 0 {
                    cost += index - empty_indices.pop_front().unwrap();
                    iter_count -= 1;
                }
                for _ in 0..iter_count {
                    overed_indices.push_back(index);
                }
            }
        }
    });

    println!("{}", cost);
}