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

fn read_stdin() -> Vec<String> {
    let mut buffer = String::new();
    io::stdin().read_to_string(&mut buffer).ok();
    buffer.trim().split('\n').map(|s| s.to_string()).collect()
}

fn main() {
    let input: Vec<usize> = read_stdin()[0].split(' ').map(|s| s.parse::<usize>().unwrap()).collect();
    let mut iter = input.iter();
    let i = iter.next();
    if i == Some(&2) {
        println!("{}", 1);
        return
    }
    for (x, y) in input.iter().zip(iter.collect::<Vec<&usize>>()) {
        if y - x != 1 {
            println!("{}", y - 1);
            return
        }
    }
    println!("{}", 10);
}