#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse4,avx,avx2,fma")
using namespace std;
const string nl = "\n";
using ll = int64_t;
using vi = vector<int>;
using vl = vector<ll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using str = string;
using vpii = vector<pii>;
using vpll = vector<pll>;
template <class T>
inline bool ckmin(T& a, T b) {
  return b < a ? a = b, 1 : 0;
}
template <class T>
inline bool ckmax(T& a, T b) {
  return b > a ? a = b, 1 : 0;
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void usaco(string x = "") {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  x = x;
  if ((int)x.size()) {
    freopen((x + ".in").c_str(), "r", stdin);
    freopen((x + ".out").c_str(), "w", stdout);
  }
}
str to_string(char c) { return str(1, c); }
str to_string(bool b) { return b ? "true" : "false"; }
str to_string(const char* second) { return (str)second; }
str to_string(str second) { return second; }
template <class A>
str to_string(complex<A> c) {
  stringstream ss;
  ss << c;
  return ss.str();
}
str to_string(vector<bool> v) {
  str res = "{";
  for (int i = 0; i < (int)v.size(); i++) res += char('0' + v[i]);
  res += "}";
  return res;
}
template <size_t SZ>
str to_string(bitset<SZ> b) {
  str res = "";
  for (int i = 0; i < b.size(); i++) res += char('0' + b[i]);
  return res;
}
template <class A, class B>
str to_string(pair<A, B> p);
template <class T>
str to_string(T v) {
  bool fst = 1;
  str res = "{";
  for (const auto& x : v) {
    if (!fst) res += ", ";
    fst = 0;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <class A, class B>
str to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
void DBG() { cerr << "]" << endl; }
template <class H, class... T>
void DBG(H h, T... t) {
  cerr << to_string(h);
  if (sizeof...(t)) cerr << ", ";
  DBG(t...);
}
int n, m, p[100050];
pii dp[100050];
str c;
set<int> q[2][2];
signed main() {
  usaco("");
  cin >> n >> c >> m;
  for (int i = (0); i < (n); i++) {
    if (c[i] == '?') {
      p[i + 1] = p[i] + 1;
    } else {
      p[i + 1] = p[i];
      q[c[i] - 'a'][i % 2].insert(i);
    }
  }
  for (int i = (0); i < (2); i++)
    for (int j = (0); j < (2); j++) q[i][j].insert(n + 50);
  if (m > n) return cout << 0 << nl, 0;
  dp[n] = {0, 0};
  for (int i = (n - 1); i >= (0); i--) {
    dp[i] = dp[i + 1];
    if (i + m <= n) {
      int x = *(q[1][i % 2].lower_bound(i));
      if (x < i + m) continue;
      x = *(q[0][(i % 2) ^ 1].lower_bound(i));
      if (x < i + m) continue;
      pii e = dp[i + m];
      e.second -= p[i + m] - p[i];
      e.first++;
      ckmax(dp[i], e);
    }
    0;
  }
  cout << -dp[0].second << nl;
}
