#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
#define rep(i,l,r) for(int i=l;i<r;i++)
#define LINF (ll)1e18
#define INF (int)1e9
struct aaa{aaa(){cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(10);};}aaaaaaa;
inline void in() {}
template <class Head, class... Tail>
inline void in(Head&& head,Tail&&... tail){
    cin>>head;
    in(move(tail)...);
}
template <class T>
inline void out(T t) {
    cout<<t<<'\n';
}
template <class Head, class... Tail>
inline void out(Head head,Tail... tail){
    cout<<head<<' ';
    out(move(tail)...);
}
int main(){
    int a,b;
    in(a,b);
    string s,t;
    in(s,t);
    int b1=accumulate(s.begin(), s.end(), 0)-'A'*a;
    int b2=accumulate(t.begin(), t.end(), 0)-'A'*b;
    out(min(a-b1,b-b2)+min(b1,b2));
}