#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, class T1>
inline void gn(T &first, T1 &second) {
  gn(first);
  gn(second);
}
template <class T, class T1, class T2>
inline void gn(T &first, T1 &second, T2 &z) {
  gn(first);
  gn(second);
  gn(z);
}
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');
}
template <class T>
inline void printsp(T first) {
  print(first);
  putchar(' ');
}
template <class T>
inline void println(T first) {
  print(first);
  putchar('\n');
}
template <class T, class U>
inline void print(T first, U second) {
  printsp(first);
  println(second);
}
template <class T, class U, class V>
inline void print(T first, U second, V z) {
  printsp(first);
  printsp(second);
  println(z);
}
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[202020];
int cnt[2][2][202020], cc[202020];
pair<int, int> ans[202020];
pair<int, int> operator+(pair<int, int> a, pair<int, int> b) {
  return pair<int, int>(a.first + b.first, a.second + b.second);
}
int can(int k, int m) {
  if (m & 1) {
    if (cnt[k & 1][1][k] - cnt[k & 1][1][k - m - 1] ||
        cnt[~k & 1][0][k - 1] - cnt[~k & 1][0][k - m])
      return 0;
  } else {
    if (cnt[k & 1][0][k] - cnt[k & 1][0][k - m] ||
        cnt[~k & 1][1][k - 1] - cnt[~k & 1][1][k - m - 1])
      return 0;
  }
  return 1;
}
int main() {
  int n, m;
  gn(n);
  scanf("%s", s + 2);
  gn(m);
  for (int i = 2; i <= n + 1; i++) {
    cnt[i & 1][0][i] = cnt[i & 1][0][i - 2] + (s[i] == 'a');
    cnt[i & 1][1][i] = cnt[i & 1][1][i - 2] + (s[i] == 'b');
    cc[i] = cc[i - 1] + (s[i] == '?');
  }
  for (int i = m + 1; i <= n + 1; i++) {
    ans[i] = ans[i - 1];
    if (can(i, m)) {
      if (ans[i].first < ans[i - m].first + 1) {
        ans[i] = ans[i - m] + pair<int, int>(1, cc[i] - cc[i - m]);
      } else if (ans[i].first == ans[i - m].first + 1) {
        smin(ans[i], ans[i - m] + pair<int, int>(1, cc[i] - cc[i - m]));
      }
    }
  }
  println(ans[n + 1].second);
  return 0;
}
