#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>

using namespace std;

int main(void) {
  int d;
  char s[111], t[111];
  memset(s, 'x', sizeof(s));
  scanf("%d", &d);
  scanf("%s", s+14);
  scanf("%s", s+14+7);
  s[14+7+7] = 'x';

  int res = 0;
  for (int i = 0; i < 77; i++) {
    if (s[i] == 'o')continue;
    memcpy(t, s, sizeof(s));

    bool o = false;
    
    for (int j = 0; j < d; j++) {
      if (s[i+j] == 'o')break;
      s[i+j] = 'o';
    }

    if (!o) {
      for (int j = 0; j < 99; j++) {
        if (s[j] == 'o') {
          int c = 1;
          while (s[j+c] == 'o') {
            c++;
          }
          res = max(res, c);
        }
      }
    }
    
    memcpy(s, t, sizeof(t));
  }

  printf("%d\n", res);
  
  return 0;
}