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

#[derive(Debug)]
struct Input {
    bs: Vec<i32>,
}

fn next_token(cin_lock: &mut io::StdinLock) -> String {
    cin_lock
        .by_ref()
        .bytes()
        .map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect::<String>()
}

fn read_input(cin_lock: &mut io::StdinLock) -> Input {
    let bs = (0..9)
        .map(|_| next_token(cin_lock).parse().unwrap())
        .collect();
    Input { bs }
}

fn solve(mut input: Input, _cin_lock: &mut io::StdinLock) {
    input.bs.sort();
    let answer = input
        .bs
        .into_iter()
        .enumerate()
        .find(|(i, b)| i + 1 != *b as usize)
        .map(|(_, b)| b - 1)
        .unwrap_or(10);
    println!("{}", answer);
}

fn main() {
    let cin = io::stdin();
    let mut cin_lock = cin.lock();

    let input = read_input(&mut cin_lock);
    solve(input, &mut cin_lock);
}