extern crate core;

use std::io::{self, Read};
use lib::*;

fn main() {
    let mut input = String::new();
    io::stdin().read_to_string(&mut input).unwrap();
    let output = run(input.trim().to_string());
    println!("{}", output);
}

fn run(input: String) -> String {
    let mut list = input
        .split_whitespace()
        .skip(1)
        .map(|x| x.parse::<i32>().unwrap())
        .collect::<Vec<_>>();
    list.sort();

    let mut a = 0;
    let mut b = 0;

    while list.len() != 0 {
        a += list.pop().unwrap();
        b += list.pop().unwrap_or(0);
    }

    (a - b).to_string()
}

#[test]
fn test() {
    let tests = vec![
        (
            "2
3 1",
            "2",
        ),
        (
            "3
2 7 4",
            "5",
        ),
        (
            "4
20 18 2 18",
            "18",
        ),
    ];
    for (i, (input, output)) in tests.into_iter().enumerate() {
        println!("test:{}", i);
        assert_eq!(run(input.to_string()), output.to_string());
    }
}

mod lib {
    //https://qiita.com/edo_m18/items/0588d290a19f2afc0a84
    mod graph {
        #[derive(Debug, PartialEq, Clone)]
        pub struct Node {
            pub edges: Vec<(usize, f64)>,
            pub done: bool,
            pub cost: f64,
            pub id: usize,
            pub previous_node: Option<usize>,
        }

        impl Node {
            pub fn new(id: usize) -> Node {
                Node {
                    edges: Vec::new(),
                    done: false,
                    cost: -1.0,
                    id: id,
                    previous_node: None,
                }
            }

            pub fn add_node(&mut self, id: usize, cost: f64) {
                self.edges.push((id, cost));
            }
        }

        pub fn min_route(mut nodes: Vec<Node>) -> (f64, Vec<Node>) {
            nodes[0].cost = 0.0;

            loop {
                let mut process_node: Option<usize> = None;

                for node in &nodes {
                    // 訪問済み or まだコストが未設定
                    if node.done || node.cost < 0.0 {
                        continue;
                    }

                    match process_node {
                        Option::None => process_node = Some(node.id),
                        Option::Some(pn) => {
                            if node.cost < nodes[pn].cost {
                                process_node = Some(node.id);
                            }
                        }
                    }
                }

                match process_node {
                    Option::Some(pn) => {
                        nodes[pn].done = true;

                        for edge in nodes[pn].edges.clone() {
                            let node = edge.0;
                            let cost = nodes[pn].cost + edge.1;

                            // コストが未設定 or コストの少ない経路がある場合はアップデート
                            let needs_update =
                                (nodes[node].cost < 0.0) || (nodes[node].cost > cost);
                            if needs_update {
                                nodes[node].cost = cost;
                                nodes[node].previous_node = Some(pn);
                            }
                        }
                    }
                    None => {
                        break;
                    }
                }
            }

            let goal_node = nodes.last().unwrap();

            let mut path: Vec<Node> = Vec::new();
            let mut current_node = goal_node;
            loop {
                let next_node = current_node.previous_node;
                match next_node {
                    Option::None => {
                        break;
                    }
                    Option::Some(next_node) => {
                        path.push(nodes[next_node].clone());
                        current_node = &nodes[next_node];
                    }
                }
            }

            let sum = path.clone()
                .into_iter()
                .filter_map(|x| {
                    x.previous_node
                        .map(|id| x.edges.iter().find(|x| x.0 == id).unwrap().1)
                })
                .sum::<f64>() + path[0].edges.last().unwrap().1;

            (sum, path)
        }
    }

}


Measured memory_bytes: 4352