#include <algorithm>
#include <fstream>
#include <iostream>
#include <limits>
#include <vector>

using namespace std;

template<class T>
class SuffixArrays {
    public:
        // Must be indexed from 0.
        explicit SuffixArrays(const vector<T>& array) :
            array_(array), array_size_(array.size()) {
            min_value_ = *min_element(array.begin(), array.end());
        }

        void Build() {
            suffix_.resize(GetLog(array_.size()));
            for (int i = 0; i < suffix_.size(); i++) {
                suffix_[i].resize(array_.size());
            }

            for (int i = 0; i < array_size_; i++) {
                suffix_[0][i] = array_[i] - min_value_;
            }

            for (int step = 1; step < suffix_.size(); step++) {
                int length = 1 << (step - 1);

                pieces_.clear();
                for (int i = 0; i < array_size_; i++) {
                    int current_left_piece = suffix_[step - 1][i];
                    int current_right_piece = (i + length < array_size_) ?
                                              suffix_[step - 1][i + length] : -1;
                    int current_index = i;
                    pieces_.emplace_back(current_left_piece, current_right_piece, current_index);
                }

                sort(pieces_.begin(), pieces_.end());

                for (int i = 0; i < array_size_; i++) {
                    if (i > 0 && pieces_[i] == pieces_[i - 1]) {
                        suffix_[step][pieces_[i].index_] = suffix_[step][pieces_[i - 1].index_];
                    } else {
                        suffix_[step][pieces_[i].index_] = i;
                    }
                }
            }

            sorted_indexes_.resize(array_size_);
            for (int i = 0; i < array_size_; i++) {
                sorted_indexes_[suffix_[suffix_.size() - 1][i]] = i;
            }
        }

        vector<int> GetSortedIndexes() {
            if (sorted_indexes_.empty()) {
                Build();
            }
            return sorted_indexes_;
        }

        int GetLongestCommonPrefix(int index_x, int index_y) {
            if (suffix_.empty()) {
                Build();
            }

            int lcp = 0;
            for (int i = suffix_.size() - 1, length = (1 << i); i >= 0
                    && index_x + length < array_size_ && index_y + length < array_size_; i--) {
                if (suffix_[i][index_x] == suffix_[i][index_y]) {
                    lcp += length;
                    index_x += length;
                    index_y += length;
                }
            }

            return lcp;
        }

    private:
        struct ArrayPiece {
            int left_piece_;
            int right_piece_;
            int index_;

            ArrayPiece(const int left_piece, const int right_piece, const int index) :
                left_piece_(left_piece), right_piece_(right_piece), index_(index) {}

            bool operator < (const ArrayPiece& other) const {
                if (this->left_piece_ == other.left_piece_) {
                    return this->right_piece_ < other.right_piece_;
                }
                return this->left_piece_ < other.left_piece_;
            }

            bool operator == (const ArrayPiece& other) const {
                return this->left_piece_ == other.left_piece_
                       && this->right_piece_ == other.right_piece_;
            }
        };

        int GetLog(int x) const {
            int log = 0;

            do {
                log++;
                x /= 2;
            } while (x);

            return log;
        }

        const int array_size_;
        const vector<T> array_;
        T min_value_;
        vector<vector<int>> suffix_;
        vector<ArrayPiece> pieces_;
        vector<int> sorted_indexes_;
};

int main() {
    cin.sync_with_stdio(false);

    ifstream cin("substr.in");
    ofstream cout("substr.out");

    int n, k;
    cin >> n >> k;

    string s;
    cin >> s;

    vector<char> v;
    for (char it : s) {
        v.push_back(it);
    }

    SuffixArrays<char> sa(v);
    sa.Build();

    auto sorted_indexes = sa.GetSortedIndexes();

    int ans = 0;
    for (int i = 0; i < n - k + 1; i++) {
        ans = max(ans, sa.GetLongestCommonPrefix(sorted_indexes[i],
                  sorted_indexes[i + k - 1]));
    }
    cout << ans << '\n';

    return 0;
}