use std::cmp::max;
use std::io::*;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let n: usize = itr.next().unwrap().parse().unwrap();
    let s: Vec<char> = itr.next().unwrap().chars().collect();
    let t: Vec<char> = itr.next().unwrap().chars().collect();

    let mut week = Vec::new();
    for _ in 0..14 {
        week.push(false);
    }
    for i in 0..7 {
        week.push(s[i] == 'o');
    }
    for i in 0..7 {
        week.push(t[i] == 'o');
    }
    for _ in 0..14 {
        week.push(false);
    }

    let mut ans = 0;
    for i in 0..42 {
        let mut d = n;
        let mut cnt = 0;
        let mut c1 = false;
        let mut c2 = false;
        for j in i..42 {
            if !week[j] {
                c1 = true;
                if c2 {
                    break;
                }
                if d != 0 {
                    cnt += 1;
                    d -= 1;
                } else {
                    break;
                }
            } else {
                if c1 {
                    c2 = true;
                }
                cnt += 1;
            }
        }
        ans = max(ans, cnt);
    }
    println!("{}", ans);
}