#include <bits/stdc++.h>
using namespace std;
template <typename T>
void out(T x) {
  cout << x << endl;
  exit(0);
}
using ll = long long;
const ll mod = 1e9 + 7;
const int maxn = 1e6 + 5;
int n, k;
string S;
bool test(int start) {
  string s = S;
  int cur = 0;
  for (int i = 0; i < start; i++) {
    if (s[i] == '?' || s[i] == 'Y') {
      s[i] = 'Y';
      cur = 0;
    } else if (s[i] == 'N') {
      cur++;
    }
    if (cur > k) return false;
  }
  if (start > 0 && s[start - 1] == 'N') {
    return false;
  }
  for (int i = start; i < start + k; i++) {
    if (s[i] == 'Y') {
      return false;
    }
  }
  if (start + k < n && s[start + k] == 'N') {
    return false;
  }
  cur = 0;
  for (int i = start + k; i < n; i++) {
    if (s[i] == '?' || s[i] == 'Y') {
      s[i] = 'Y';
      cur = 0;
    } else if (s[i] == 'N') {
      cur++;
    }
    if (cur > k) return false;
  }
  return true;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> k;
  cin >> S;
  for (int i = 0; i + k - 1 < n; i++) {
    if (test(i)) {
      out("YES");
    }
  }
  cout << "NO\n";
  return 0;
}
