use std::collections::BTreeMap;


fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();
    let a = (0..n).map(|_| {
            let mut temp = String::new();
            std::io::stdin().read_line(&mut temp).ok();
            let temp: usize = temp.trim().parse().unwrap();
            temp
        })
        .collect::<Vec<_>>();

    let mut mapping = BTreeMap::new();
    for &v in a.iter() { *mapping.entry(v).or_insert(0usize) += 1; }
    let mut prev = 0usize;
    let mut prev_cnt = 0usize;
    let mut result = 0usize;
    for (&val, &cnt) in mapping.iter() {
        if prev + 1 == val {
            result = result.max(prev_cnt + cnt);
        } else {
            result = result.max(cnt);
        }
        prev = val;
        prev_cnt = cnt;
    }
    println!("{}", result);
}