fn main() {
    let mut xx = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut xx).ok();
    let xx: Vec<&str> = xx.split_whitespace().skip(2).collect();

    let (a_s, a_t) = (xx[0].matches('A').count(), xx[1].matches('A').count());
    let (b_s, b_t) = (xx[0].matches('B').count(), xx[1].matches('B').count());

    println!("{}", a_s.min(a_t) + b_s.min(b_t));
}