use std::cmp::min;

fn main() {
    let mut nm = String::new();
    std::io::stdin().read_line(&mut nm).ok();
    let nm: Vec<usize> = nm.trim().split_whitespace().map(|i| i.parse().unwrap()).collect();
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s: &str = s.trim();
    let mut t = String::new();
    std::io::stdin().read_line(&mut t).ok();
    let t: &str = t.trim();
    let sa: usize = s.chars().filter(|i| *i == 'A').count();
    let ta: usize = t.chars().filter(|i| *i == 'A').count();

    println!("{}", min(sa, ta) + min(s.len() - sa, t.len() - ta));
}