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

using ll = long long;
using db = double;
mt19937_64 rng(time(0));
ll get(ll l, ll r) {
    return uniform_int_distribution<ll>(l, r)(rng);
}
constexpr db eps = 1e-9;

void solve() {
    int d, w;
    cin >> d >> w;
    vector<string> roll(d);
    set<string> ws;
    int S = 1 << (3 * d);
    vector G(S, vector<int>());
    for(int i = 0; i < d; i++) {
        cin >> roll[i];
    }
    for(int i = 0; i < w; i++) {
        string s;
        cin >> s;
        sort(s.begin(), s.end());
        ws.insert(s);
    }
    int flg = 0;
    vector<int> states;
    vector<db> f(S), g(S);
    vector<int> count(S), pw6(d + 1);
    pw6[0] = 1;
    for(int i = 1; i <= d; i++) pw6[i] = pw6[i - 1] * 6;
    for(int i = 1; i < (S); i++) {
        count[i] = count[i >> 3] + ((i & 7) != 0);
    }
    auto dfs = [&](auto self, int x, string t, int cur) {
        if(x == d) {
            sort(t.begin(), t.end());
            states.push_back(cur);
            if(ws.count(t)) flg = 1;
            else f[cur] = get(9e8, 1e9);
            for(int i = 1; i < (1 << d); i++) {
                int to = cur;
                for(int j = 0; j < d; j++) if(i >> j & 1) {
                    to &= ~(7 << (3 * j));
                }
                G[cur].push_back(to);
            }
            return;
        }
        for(int i = 1; i <= 6; i++) {
            self(self, x + 1, t + roll[x][i - 1], cur | (i << (3 * x)));
        }
    };
    dfs(dfs, 0, "", 0);
    // cerr << "ok\n";
    if(!flg) {
        cout << "impossible\n";
        return;
    }
    double delta = 0;
    // for(auto st : states) {
    //     // ans += f[st];
    //     // cerr << f[st] << '\n';
    //     cerr << st << ": ";
    //     for(auto nst : G[st]) {
    //         cerr << nst << ' ';
    //     }
    //     cerr << '\n';
    // }
    // // return;
    do {
        g.assign(S, 0);
        for(auto st : states) {
            for(auto nst : G[st]) {
                g[nst] += f[st];
            }
        }
        for(int i = 0; i < S; i++) {
            assert(count[i] <= d && count[i] >= 0);
            g[i] /= pw6[d - count[i]];
            // cerr << g[i] << '\n';
        }
        delta = 0;
        for(auto st : states) if(f[st] >= eps) {
            double nxt = 1e9;
            for(auto nst : G[st]) {
                nxt = fmin(nxt, g[nst]);
            }
            nxt += 1;
            // cerr << nxt << ' ' << fabs(nxt - f[st]) << '\n';
            delta += fabs(nxt - f[st]);
            f[st] = nxt;
        }
        // cerr << "-----------------\n";
        // break;
    } while(delta >= eps);
    db ans = 0;
    for(auto st : states) {
        ans += f[st];
        // cerr << f[st] << '\n';
    }
    cout << setprecision(20) << ans / pw6[d] + 1 << '\n';
}
signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    int t = 1;
    // cin >> t;
    while(t--) solve();
    return 0;
}