#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, U b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, U b) {
  if (a < b) a = b;
}
template <class T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T>
inline void print(T first) {
  if (first < 0) {
    putchar('-');
    return print(-first);
  }
  if (first < 10) {
    putchar('0' + first);
    return;
  }
  print(first / 10);
  putchar(first % 10 + '0');
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = 1LL * a * a % m)
    if (b & 1) ans = 1LL * ans * a % m;
  return ans;
}
char s[100011];
pair<int, int> dp[100011];
int cnt[100011], mx[100011][2];
int main() {
  int n, m;
  cin >> n >> s + 1 >> m;
  for (int i = 1; i <= n; i++) {
    if (s[i] != 'b') mx[i][0] = mx[i - 1][1] + 1;
    if (s[i] != 'a') mx[i][1] = mx[i - 1][0] + 1;
    cnt[i] = cnt[i - 1] + (s[i] == '?');
    dp[i] = dp[i - 1];
    if (mx[i][!(m & 1)] >= m) {
      if (dp[i - m].first + 1 == dp[i].first) {
        smin(dp[i].second, dp[i - m].second + cnt[i] - cnt[i - m]);
      }
      if (dp[i - m].first + 1 > dp[i].first) {
        dp[i].first = dp[i - m].first + 1;
        dp[i].second = dp[i - m].second + cnt[i] - cnt[i - m];
      }
    }
  }
  cout << dp[n].second << endl;
  return 0;
}
