#include "bits/stdc++.h"

using namespace std;

int main() {
  int N, M;
  cin >> N >> M;
  string S, T;
  cin >> S >> T;
  cout << min(count(begin(S), end(S), 'A'), count(begin(T), end(T), 'A')) + min(count(begin(S), end(S), 'B'), count(begin(T), end(T), 'B')) << endl;
}