Untitled

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

// Multiset with less_equal
typedef tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;
void PPPPPP() {
    ios::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
}
#define  int long long
const int mod = 1e9 + 7;
long long power(int x, int y, int p) {
    int result = 1;
    x = x % p;
    while (y > 0) {
        if (y & 1)
            result = (result * x) % p;
        y = y >> 1;
        x = (x * x) % p;
    }
    return result;
}
int mod_inverse(int a, int p) {
    return power(a, p - 2, p);
}
#define all(a) a.begin(), a.end()

int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, 1, - 1};
signed main() {
    PPPPPP();
    int t = 1;
//    cin >> t;
    Z: while (t--)  {
      int n, k; cin >> n >> k;
      string s; cin >> s;
      int pre[26][n + 1]; memset(pre, 0 , sizeof pre);
      for (int i = 0; i < n; i++){
          pre[s[i]- 'a'][i + 1] = 1;
      }
      for (int i = 0; i < 26; i++) for (int j = 1; j <= n; j++) pre[i][j] += pre[i][j - 1];
      int ans = 0;
      for (int i = 0; i + k <= n; i++){
          int cur = 0;
          for(auto ch = 'a'; ch <= 'z'; ch++){
              if (pre[ch - 'a'][i + k] - pre[ch - 'a'][i]) cur++;
          }
          ans = max(ans, cur);
      }
      cout << ans;


}
}