#[allow(unused_macros)]
macro_rules! input {
    ( $($t:ty),* ) => {{
        let mut s = String::new();
        std::io::stdin().read_line(&mut s);
        let mut splits = s.trim().split_whitespace();
        ($( { splits.next().unwrap().parse::<$t>().unwrap() },)*)
    }}
}

#[allow(unused_must_use)]
#[allow(unused_variables)]
fn solve() {
    let (n, ) = input!(usize);
    let (s, ) = input!(String);

    let mut count = 0;
    let mut snew = "".to_string();
    for c in s.chars() {
        match c {
            '3' | '5' | '7' => count += 1,
            _ => snew.push(c),
        }
    }

    let mut count1 = 0;
    for c in snew.chars() {
        if c == '1' {
            count1 += 1;
        }
    }

    println!("{}", count + (count1 / 2));
}

fn main() {
    solve();
}