#include <bits/stdc++.h>
using namespace std;
int f(string s) {
  int res = 0;
  int cur = 0;
  for (int i = 0; i < s.size(); i++)
    if (s[i] == 'N')
      cur++;
    else {
      res = max(res, cur);
      cur = 0;
    }
  return max(res, cur);
}
int main() {
  int n, k;
  string s;
  cin >> n >> k >> s;
  for (int i = 0; i < n; i++) {
    string b = s;
    for (int j = 0; j < i; j++)
      if (b[j] == '?') b[j] = 'Y';
    for (int j = i; j < i + k; j++)
      if (b[j] == '?') b[j] = 'N';
    for (int j = i + k; j < n; j++)
      if (b[j] == '?') b[j] = 'Y';
    if (f(b) == k) {
      cout << "YES";
      return 0;
    }
  }
  cout << "NO";
}
