#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)
#define all(x) x.begin(),x.end()
int N,M;
string s,t;

int main(){
  cin >> N >> M;
  cin >> s >> t;
  int a = min(count(all(s),'A'),count(all(t),'A'));
  int b = min(count(all(s),'B'),count(all(t),'B'));
  cout << a + b << endl;
}