#include <iostream>
#include <algorithm>
#include <vector>
#include <string>

void solve() {
    {
        int n, m;
        std::cin >> n >> m;
    }

    std::vector<std::vector<int>> cnt(2, std::vector<int>(2, 0));
    for (int i = 0; i < 2; ++i) {
        std::string s;
        std::cin >> s;

        for (int j = 0; j < 2; ++j) {
            cnt[i][j] = std::count(s.begin(), s.end(), 'A' + j);
        }
    }

    int ans = 0;
    for (int j = 0; j < 2; ++j) {
        ans += std::min(cnt[0][j], cnt[1][j]);
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}