#include <bits/stdc++.h>
using namespace std;

#define int long long
#define f(i,a,b) for(int i = (a); i < (b); ++i)

struct State {
    vector<int> next = vector<int>(26, -1);
    int count = 0;
};

struct  Trie {
    vector<State> states = vector<State> (1);

    void Construct(vector<string> vals) {
        for (auto &&val : vals) {
            int state = 0;
            for (char c : val) {
                if (states[state].next[c - 'a'] == -1) {
                    states.push_back(State());
                    states[state].next[c - 'a'] = states.size() - 1;
                }
                state = states[state].next[c - 'a'];
            }
        }
    }

    int Add(string val) {
        int ret = 0;
        int state = 0;
        for (char c : val) {
            state = states[state].next[c - 'a'];
            ret += states[state].count++;
        }
        return ret;
    }

    int Remove(string val) {
        int ret = 0;
        int state = 0;
        for (char c : val) {
            state = states[state].next[c - 'a'];
            ret += --states[state].count;
        }
        return ret;
    }

};

signed main() {
    ios_base::sync_with_stdio(0);
    int n, k; cin >> n >> k;
    vector<string> vals(n);
    f(i,0,n) cin >> vals[i];


    auto t = Trie();
    t.Construct(vals);
    int ans = 0;
    int score = 0;
    int lptr = 0;
    t.Add(vals[0]);

    f(i,1,n) {
        score += t.Add(vals[i]);
        while (score >= k) score -= t.Remove(vals[lptr++]);
        ans += lptr;
    }

    cout << ans << "\n";
    return 0;

}