mod p1468 {
    use std::collections::BTreeMap;

    fn gather(v: Vec<String>) -> BTreeMap<String, usize> {
        v.into_iter().fold(BTreeMap::new(), |mut acc, s| {
            let count = acc.get(&s).unwrap_or(&0);
            acc.insert(s, count + 1);
            acc
        })
    }

    pub fn solve(begin: Vec<String>, after: Vec<String>) -> String {
        let begin_map = gather(begin);
        let after_map = gather(after);

        for (k, v) in begin_map.into_iter() {
            if let Some(taken) = after_map.get(&k) {
                if taken != &v {
                    return k
                }
            } else {
                return k
            }
        }
        return "--".to_string()
    }
}

struct Input {
    num: usize,
    begin: Vec<String>,
    after: Vec<String>,
}

impl Input {
    fn readline(len: usize) ->  Vec<String> {
        use std::io::{self, BufRead};

        let stdin = io::stdin();
        let lines = stdin.lock().lines().map(|l| l.unwrap().to_string());
        lines.take(len).collect()
    }

    fn accept(len: usize) -> Self {
        let lines = Self::readline(len);
        let num = lines[0].parse::<usize>().unwrap();
        let begin = lines[1].split(" ").map(|s| s.to_string()).collect::<Vec<_>>();
        let after = lines[2].split(" ").map(|s| s.to_string()).collect::<Vec<_>>();
        Self {
            num,
            begin,
            after,
        }
    }
}

fn main() {
    let args = Input::accept(3);
    let result = p1468::solve(args.begin, args.after);
    println!("{}", result);
}