#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;
int main() {
  int N,M; cin >> N >> M;
  string A; cin >> A;
  string B; cin >> B;
  sort(A.begin(),A.end());
  sort(B.begin(),B.end());
  int ans = 0;
  for(int i = 0; i < A.size(); i++) {
    if(i < M && A[i] == B[i]) {
      ans++;
    }
  }
  cout << ans << endl;
}