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

int main(){
  string pad = string(14, 'x');
  
  int D;
  cin >> D;
  
  string S1, S2;
  cin >> S1 >> S2;
  
  string S = pad + S1 + S2 + pad;
  
  vector<int> blocks;
  int acc = 1;
  for(int i=1;i<S.size();i++){
    if(S[i-1] == S[i]) {
      ++acc;
      continue;
    }
    blocks.push_back(acc);
    acc = 1;
  }
  blocks.push_back(acc);
  
  int res = D;
  for(int i=1;i<(int)blocks.size()-1;i+=2) {
    res = max(res, blocks[i] + min(blocks[i-1], D));
    res = max(res, blocks[i] + min(blocks[i+1], D));
  }
  for(int i=2;i<(int)blocks.size()-2;i+=2) {
    if(blocks[i] <= D) {
      res = max(res, blocks[i-1] + blocks[i] + blocks[i+1]);
    }
  }
  cout << res << endl;
  return 0;
}