#include <bits/stdc++.h>

using namespace std;

const int maxn = 5 * 1e6 + 11;

struct myset {
  vector<int> cnt;
  int mx = 0;
  myset() {
    cnt.resize(maxn);
  }
  int get_max() {
    return mx;
  }
  void decrease(int x) {
    if (x == mx && cnt[x] == 1)
      mx -= 1;
    if (x >= 0)
      cnt[x]--;
    if (x > 0)
      cnt[x - 1]++;
  }
  void increase(int x) {
    if (x == mx)
      mx += 1;
    if (x >= 0)
      cnt[x]--;
    cnt[x + 1]++;
  }
};

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
#ifdef LOCAL
  freopen("input", "r", stdin);
#endif
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  int ans = 0;
  int r = 0;
  int cnt_ch['z' + 1], cnt_nch['z' + 1];
  memset(cnt_ch, 0, sizeof cnt_ch), memset(cnt_nch, 0, sizeof cnt_nch);
  myset ch, nch;
  ch.mx = 1;
  ch.cnt[1] = 1;
  cnt_ch[s[0]] += 1;
  auto calc = [&](int l, int r) {
    return (r - l + 1) - ch.get_max() - nch.get_max();
  };
  for (int l = 0; l < n; l++) {
    while (calc(l, r) <= k) {
      r += 1;
      if (r == n)
        break;
      if (r % 2) {
        nch.increase(cnt_nch[s[r]]++);
      } else {
        ch.increase(cnt_ch[s[r]]++);
      }
    }
    if (r % 2) {
      nch.decrease(cnt_nch[s[r]]--);
    } else {
      ch.decrease(cnt_ch[s[r]]--);
    }
    r -= 1;
    ans = max(ans, r - l + 1);
    if (l % 2) {
      nch.decrease(cnt_nch[s[l]]--);
    } else {
      ch.decrease(cnt_ch[s[l]]--);
    }
  }
  cout << ans << '\n';
}