#include <iostream>
#include <vector>
using namespace std;
int main(){
  int d;cin>>d;
  string a,b;cin>>a>>b;
  a+=b;
  int ans = 0;
  for(int i = -d; 14 >= i; i++){
    int tmp = 0;
    int t = -i;
    for(int j = 0; 14 > j; j++){
      if(i <= j && j < i+d){
        t++;
      }else if(a[j] == 'o'){
        t++;
      }else{
        tmp = max(tmp,t);
        t = 0;
      }
    }
    t+=i+d-14;
    tmp = max(tmp,t);
    ans = max(ans,tmp);
  }
  cout << ans << endl;
}