fn main() {
    let _n = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().parse::<usize>().unwrap()
    };
    let aa: Vec<_> = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.split_whitespace()
            .map(|x| x.parse::<usize>().unwrap() - 1)
            .collect()
    };

    let ans = aa[0] + aa.windows(2).map(|w| w[1] - w[0] - 1).sum::<usize>();
    println!("{}", ans);
}