#include <bits/stdc++.h>
using namespace std;
template <class T, class S>
ostream& operator<<(ostream& os, const pair<T, S>& v) {
  return os << "(" << v.first << ", " << v.second << ")";
}
template <class T>
ostream& operator<<(ostream& os, const vector<T>& v) {
  os << "[";
  for (int i = int(0); i <= int((static_cast<int>((v).size())) - 1); ++i) {
    if (i) os << ", ";
    os << v[i];
  }
  return os << "]";
}
template <class T>
bool setmax(T& _a, T _b) {
  if (_b > _a) {
    _a = _b;
    return true;
  }
  return false;
}
template <class T>
bool setmin(T& _a, T _b) {
  if (_b < _a) {
    _a = _b;
    return true;
  }
  return false;
}
template <class T>
T gcd(T _a, T _b) {
  return _b == 0 ? _a : gcd(_b, _a % _b);
}
struct Node {
  vector<Node*> ys;
  int h = -1;
};
int main() {
  int n, k;
  cin >> n >> k;
  vector<string> ss(n);
  for (int i = int(0); i <= int((n)-1); ++i) cin >> ss[i];
  sort((ss).begin(), (ss).end());
  vector<int> height(n - 1);
  for (int i = int(0); i <= int((n - 1) - 1); ++i) {
    int j = 0;
    while (j < static_cast<int>((ss[i]).size()) &&
           j < static_cast<int>((ss[i + 1]).size()) &&
           ss[i][j] == ss[i + 1][j]) {
      ++j;
    }
    height[i] = j;
  }
  const function<Node*(int, int)> build = [&](int l, int r) {
    Node* x = new Node();
    if (l + 1 == r) return x;
    int h = *min_element(height.begin() + l, height.begin() + r - 1);
    x->h = h;
    int i = l;
    for (int j = int(l); j <= int((r)-1); ++j) {
      if (j == r - 1 || height[j] == h) {
        x->ys.push_back(build(i, j + 1));
        i = j + 1;
      }
    }
    return x;
  };
  Node* root = build(0, n);
  const function<vector<int>(Node*)> solve = [&](Node* x) {
    if (x->h < 0) return vector<int>{0, 0};
    vector<int> ans{0};
    for (Node* y : x->ys) {
      vector<int> tmp = solve(y);
      vector<int> r(static_cast<int>((ans).size()) +
                    static_cast<int>((tmp).size()) - 1);
      for (int i = int(0); i <= int((static_cast<int>((ans).size())) - 1); ++i)
        for (int j = int(0); j <= int((static_cast<int>((tmp).size())) - 1);
             ++j) {
          setmax(r[i + j], ans[i] + tmp[j] + i * j * x->h);
        }
      ans = r;
    }
    if (static_cast<int>((ans).size()) > k + 1) ans.resize(k + 1);
    return ans;
  };
  vector<int> ans = solve(root);
  if (k < static_cast<int>((ans).size())) {
    cout << ans[k] << endl;
  } else {
    cout << 0 << endl;
  }
  return 0;
}
