#include<iostream>
#include<algorithm>
using namespace std;

int main(){
    int n, m;
    cin >> n >> m;
    string s, t;
    cin >> s >> t;
    cout << min(count(s.begin(), s.end(), 'A'), count(t.begin(), t.end(), 'A')) + min(count(s.begin(), s.end(), 'B'), count(t.begin(), t.end(), 'B')) << endl;
    return 0;
}