fn main() {
    let stdin = std::io::read_to_string(std::io::stdin().lock()).unwrap();
    let mut stdin = stdin.split_ascii_whitespace();

    let n: usize = stdin.next().unwrap().parse().unwrap();
    let x: Vec<u32> = (0..n)
        .map(|_| stdin.next().unwrap().parse().unwrap())
        .collect();

    use std::io::Write;
    std::io::stdout()
        .lock()
        .write_all(output(solve(x)).as_bytes())
        .unwrap();
}

fn solve(mut x: Vec<u32>) -> bool {
    x.sort_unstable();
    x[0] != x[1] && x.windows(3).all(|x| x[0] + x[2] == x[1] << 1)
}

fn output(ans: bool) -> &'static str {
    match ans {
        true => "YES",
        false => "NO",
    }
}