use std::io;

fn read() -> String {
    let mut s = String::new();
    io::stdin().read_line(&mut s).ok();
    s.trim().to_string()
}

fn main() {
    let mut nm = String::new();
    io::stdin().read_line(&mut nm).ok();

    let s = read();
    let t = read();

    println!("{}",
        std::cmp::min(s.match_indices("A").count(), t.match_indices("A").count())
        + std::cmp::min(s.match_indices("B").count(), t.match_indices("B").count())
    );
}