#include <bits/stdc++.h>

typedef long long ll;

using namespace std;

struct Node{
    vector<ll> next;
    ll count = 0;
    Node () :next(26) {}
};

int main(){

    vector<Node> nodes;
    Node start;
    nodes.emplace_back(start);

    ll n, k;
    cin >> n >> k;

    ll cur = 0;
    ll ans = 0;
    ll startP = 0;

    vector<string> arr(n);

    for(ll i = 0; i < n; i++){
        string s;
        cin >> s;
        arr[i] = s;
    }

    for(ll i = 0; i < n; i++){
        string s = arr[i];
        ll curNode = 0;
        for(ll j = 0; j < s.length(); j++){
            if(nodes[curNode].next[s[j] - 'a'] == 0){
                Node newNode;
                newNode.count = 1;
                nodes[curNode].next[s[j] - 'a'] = nodes.size();
                nodes.emplace_back(newNode);
                curNode = nodes.size() - 1;
            }else{
                ll next = nodes[curNode].next[s[j] - 'a'];
                cur -= (nodes[next].count * (nodes[next].count - 1)) / 2;
                nodes[next].count++;
                cur += (nodes[next].count * (nodes[next].count - 1)) / 2;
                curNode = next;
            }
        }
        if(cur >= k) {
            while(cur >= k){
                ans += n - i;
                string s = arr[startP];
                ll curNode = 0;
                for(ll j = 0; j < s.length(); j++){
                    ll next = nodes[curNode].next[s[j] - 'a'];
                    cur -= (nodes[next].count * (nodes[next].count - 1)) / 2;
                    nodes[next].count--;
                    cur += (nodes[next].count * (nodes[next].count - 1)) / 2;
                    curNode = next;
                }
                startP++;
            }
        }
    }

    cout << ans << endl;

    return 0;
}