#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n, m;
  cin >> n >> m;

  string s, t;
  cin >> s >> t;

  int ret = 0;
  ret += min(count(ALL(s), 'A'), count(ALL(t), 'A'));
  ret += min(count(ALL(s), 'B'), count(ALL(t), 'B'));
  cout << ret << endl;
  
  return 0;
}