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

fn get_lines() -> Vec<String> {
    let stdin = io::stdin();
    let lines: Vec<String> = stdin.lock().lines().map(|l| l.unwrap()).collect();
    return lines;
}

fn main(){
    let s = &get_lines();
    let mut c = 1;
    for x in s[1].split(' ').map(|x| x.parse::<i32>().unwrap()) {
        if x != c {
            println!("{}", x - 1);
            return;
        }
        c += 1;
    }
    println!("0");
}