Problem:
Score : 300 points

Problem Statement

N persons are standing in a row. The height of the i-th person from the front is A_i.

We want to have each person stand on a stool of some heights - at least zero - so that the following condition is satisfied for every person:

Condition: Nobody in front of the person is taller than the person. Here, the height of a person includes the stool.

Find the minimum total height of the stools needed to meet this goal.

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 \ldots A_N

Output

Print the minimum total height of the stools needed to meet the goal.

Sample Input 1

5
2 1 5 4 3

Sample Output 1

4

If the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively, their heights will be 2, 2, 5, 5, and 5, satisfying the condition.

We cannot meet the goal with a smaller total height of the stools.

Sample Input 2

5
3 3 3 3 3

Sample Output 2

0

Giving a stool of height 0 to everyone will work.

Solution in Rust (1.42.0) (judge verdict: Accepted)

#![allow(dead_code, unused_imports)]

use std::fmt::Debug;
use std::str::FromStr;

fn read_stdin() -> String {
    let mut s = String::new();
    std::io::stdin()
        .read_line(&mut s)
        .expect("cannot read stdin");
    s.trim().to_string()
}

fn read<T>() -> T
where
    T: FromStr,
    <T as FromStr>::Err: Debug,
{
    read_stdin().parse::<T>().unwrap()
}

fn read_usize() -> usize {
    read::<usize>()
}

fn read_2<A, B>() -> (A, B)
where
    A: FromStr,
    <A as FromStr>::Err: Debug,
    B: FromStr,
    <B as FromStr>::Err: Debug,
{
    let s = read_stdin();
    let mut s = s.split_whitespace();
    let a = s.next().unwrap().parse::<A>().unwrap();
    let b = s.next().unwrap().parse::<B>().unwrap();
    (a, b)
}

fn read_3<A, B, C>() -> (A, B, C)
where
    A: FromStr,
    <A as FromStr>::Err: Debug,
    B: FromStr,
    <B as FromStr>::Err: Debug,
    C: FromStr,
    <C as FromStr>::Err: Debug,
{
    let s = read_stdin();
    let mut s = s.split_whitespace();
    let a = s.next().unwrap().parse::<A>().unwrap();
    let b = s.next().unwrap().parse::<B>().unwrap();
    let c = s.next().unwrap().parse::<C>().unwrap();
    (a, b, c)
}

fn read_4<A, B, C, D>() -> (A, B, C, D)
where
    A: FromStr,
    <A as FromStr>::Err: Debug,
    B: FromStr,
    <B as FromStr>::Err: Debug,
    C: FromStr,
    <C as FromStr>::Err: Debug,
    D: FromStr,
    <D as FromStr>::Err: Debug,
{
    let s = read_stdin();
    let mut s = s.split_whitespace();
    let a = s.next().unwrap().parse::<A>().unwrap();
    let b = s.next().unwrap().parse::<B>().unwrap();
    let c = s.next().unwrap().parse::<C>().unwrap();
    let d = s.next().unwrap().parse::<D>().unwrap();
    (a, b, c, d)
}

fn read_multiple<T: FromStr>() -> Vec<T>
where
    <T as FromStr>::Err: Debug,
{
    read_stdin()
        .split_whitespace()
        .map(|x| x.parse::<T>().expect("cannot parse stdin"))
        .collect()
}

/// NOTE: sort iter beforehand if required
fn count<T>(iter: &mut impl Iterator<Item = T>) -> Vec<(T, usize)>
where
    T: std::cmp::Ord,
{
    let iter = iter.collect::<Vec<_>>();
    //iter.sort();
    let (mut v, o, c) =
        iter.into_iter()
            .fold((Vec::new(), None, 0), |(mut v, last, count), item| {
                if let Some(o) = last {
                    if item == o {
                        (v, Some(o), count + 1)
                    } else {
                        v.push((o, count));
                        (v, Some(item), 1)
                    }
                } else {
                    (v, Some(item), 1)
                }
            });
    if let Some(i) = o {
        v.push((i, c));
    }
    v
}

fn max_subarray(v: impl Iterator<Item = i64>) -> (i64, (usize, usize)) {
    //assert!(v.len() > 0);
    let mut best_sum = 0;
    let (mut best_start, mut best_end) = (0, 0);
    let mut current_sum = 0;
    let mut current_start = 0;
    for (end, val) in v.enumerate() {
        if current_sum <= 0 {
            current_start = end;
            current_sum = val;
        } else {
            current_sum += val;
        }
        if current_sum > best_sum {
            best_sum = current_sum;
            best_start = current_start;
            best_end = end + 1;
        }
    }
    (best_sum, (best_start, best_end))
}

fn gcd(a: u64, b: u64) -> u64 {
    let mut a = a;
    let mut b = b;
    while a != b {
        if a > b {
            a = a - b
        } else {
            b = b - a
        }
    }
    a
}

fn factorize(mut n: u64) -> Vec<u64> {
    if n <= 3 {
        return vec![n];
    }
    let mut v = Vec::new();
    while n % 2 == 0 {
        n /= 2;
        v.push(2);
    }
    while n % 3 == 0 {
        n /= 3;
        v.push(3);
    }
    let mut f = 6;
    while (f - 1) * (f - 1) <= n {
        while n % (f - 1) == 0 {
            n /= f - 1;
            v.push(f - 1);
        }
        while n % (f + 1) == 0 {
            n /= f + 1;
            v.push(f + 1);
        }
        f += 6;
    }
    if n > 1 {
        v.push(n);
    }
    v
}

fn compact_factors(n: u64) -> Vec<(u64, usize)> {
    count(&mut factorize(n).into_iter())
}

fn all_factors(n: u64) -> Vec<u64> {
    let factors = compact_factors(n);
    let mut v = vec![1];
    for (fac, num) in factors {
        let ori = v.clone();
        for i in 1..num + 1 {
            v.append(
                &mut ori
                    .clone()
                    .into_iter()
                    .map(|f| f * fac.pow(i as u32))
                    .collect::<Vec<_>>(),
            )
        }
    }
    v.sort();
    v
}

use std::cmp::{max, min};
use std::collections::VecDeque;

fn first_factor(n: u32) -> u32 {
    if n % 2 == 0 {
        return 2;
    }
    if n % 3 == 0 {
        return 3;
    }
    let mut f = 6;
    while (f - 1) * (f - 1) <= n {
        if n % (f - 1) == 0 {
            return f - 1;
        }
        if n % (f + 1) == 0 {
            return f + 1;
        }
        f += 6;
    }
    return n;
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let _n = read_usize();
    let v = read_multiple::<u64>();
    let mut max = 0;
    let mut stool = 0;
    for i in v {
        if i >= max {
            max = i
        } else {
            stool += max - i;
        }
    }
    println!("{}", stool);
    Ok(())
}