use std::io;
fn main() {
    let mut now = 1;
    let mut line = String::new();
    io::stdin().read_line(&mut line);
    for i in line.split_whitespace() {
        if i.parse::<u32>().unwrap() != now {
            break;
        }
        now += 1;
    }
    println!("{}", now);
}