#include <bits/stdc++.h>
using namespace std;

int main() {
  int N, M, A, B, C, D;
  string S, T;
  cin >> N >> M >> S >> T;
  A = count(S.begin(), S.end(), 'A');
  B = count(T.begin(), T.end(), 'A');
  C = count(S.begin(), S.end(), 'B');
  D = count(T.begin(), T.end(), 'B');
  cout << min(A, B) + min(C, D) << "\n";
}