use std::io::Read;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_to_string(&mut s).ok();
	let s: Vec<_> = s.lines().collect();
	let n: Vec<usize> = s[0].split_whitespace().flat_map(str::parse).collect();
	let t = s[1].chars().filter(|&c| c == 'A').count();
	let b = s[2].chars().filter(|&c| c == 'A').count();
	println!("{}", t.min(n[0] - t) + b.min(n[1] - b))
}