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 n: i32 = s[0].parse().unwrap();
    let a: Vec<i32> = s[1].split(' ').map(|x| x.parse().unwrap()).rev().collect();
    let mut c = a[0];
    for x in &a[1..] {
        if *x == c - 1 {
            c -= 1;
        }
    }
    println!("{}", n - (a[0] - c));
}