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); }