fn main() {
    proconio::input! {
        n: usize,
        a: [i64; n],
    }
    let is_possible_from_front = a
        .iter()
        .scan(0, |acc, &x| {
            *acc = x.max(*acc - 1);
            Some(*acc <= 1)
        })
        .collect::<Vec<_>>();
    let is_possible_from_back = a
        .iter()
        .rev()
        .scan(0, |acc, &x| {
            *acc = x.max(*acc - 1);
            Some(*acc <= 1)
        })
        .collect::<Vec<_>>();
    if (0..n).any(|idx| is_possible_from_front[idx] && is_possible_from_back[idx]) {
        println!("Yes");
    } else {
        println!("No");
    }
}