#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define FOR(i, j, k, in) for (int i=j; i<k; i+=in)
#define RFOR(i, j, k, in) for (int i=j; i>=k; i-=in)
#define REP(i, j) FOR(i, 0, j, 1)
#define RREP(i, j) FOR(i, j, 0, 1)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.end(), a.begin()
#define F first
#define S second
#define PB push_back
#define MP make_pair
#define pii pair<int, int>
#define MOD 1000000007

ll total=0;

struct trie {
    int words=0;
    int ends=0;
    vector<trie*> sons = vector<trie*>(26, NULL);
};

void insertw(string a, trie * t) {
    t->words=0;
    REP(i, a.size()) {
        total += t->words;
        t->words++;
        if (t->sons[a[i] - 'a'] == NULL) {
            t->sons[a[i] - 'a'] = new trie;
        }
        t = t->sons[a[i] - 'a'];
    }
    total += t->words;
    t->words++;
    t->ends++;
}

void deletew(string a, trie * t) {
    t->words=1;
    REP(i, a.size()) {
        t->words--;
        total -= t->words;
        t = t->sons[a[i] - 'a'];
    }
    t->words--;
    total -= t->words;
    t->ends--;
}

void solve() {
    ll n, k; cin >> n >> k;
    vector<string> st(n);
    ll m=0;
    REP(i, n) {
        cin >> st[i];
        m = max(m, (ll) st[i].size());
    }
    int s=0, e=0;
    ll res=0;
    trie t;
    while (true) {
        if (total < k) {
            if (e == n) {
                break;
            }
            insertw(st[e], &t);
            e++;
        } else {
            // cout << s << e << endl;
            deletew(st[s], &t);
            s++;
            res += n-e+1;
        }
        // cout << total << endl;
        // REP(i, 26) {
        //     // cout << (char)('a' + i);
        // }
        // // cout << endl;
        // REP(j, m) {
        //     REP(i, 26) {
        //         // cout << totals[j][i];
        //     }
        //     // cout << endl;
        // }
    }
    cout << res << endl;


}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    ll n=1;
    //  cin >> n;
    REP(i, n) {
        solve();
    }
    return 0;
}