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

typedef unsigned long long ull;
typedef long double ld;

const int N = 125000;

priority_queue < pair <ld, int> > Q;
vector <int> son[N], par[N];
ld sum[N], dis[N];
int cnt[N], fix[N], cnt0[N];
int pw[10], pw6[10], id[10];
char c[10][10], foo[10], tmp[10];
int n, m;

set <ull> str;

ull hs(char *c) {
	ull ans = 0, pw = 1;
	for (int i = 1; i <= 6; i++) {
		ans += c[i] * pw;
		pw *= 300;
	}
	return ans;
}

int get(int x, int i) {
	return (x % pw[i]) / pw[i - 1];
}

int main() {
	scanf("%d%d", &n, &m);
	for (int i = 1; i <= n; i++) {
		scanf("%s", c[i] + 1);
	}
	for (int i = 1; i <= m; i++) {
		scanf("%s", foo + 1);
		str.insert(hs(foo));
	}
	pw[0] = pw6[0] = 1;
	for (int i = 1; i <= n; i++) {
		pw[i] = pw[i - 1] * 7;
		pw6[i] = pw6[i - 1] * 6;
	}
	vector <int> good;
	for (int i = 0; i < pw[n]; i++) {
		int bad = 0;
		for (int j = 1; j <= n; j++) {
			int now = get(i, j);
			if (!now) {
				++bad;
			}
			tmp[j] = c[j][now];
			id[j] = j;
		}
		cnt0[i] = bad;
		if (bad) continue;
		do {
			for (int j = 1; j <= n; j++) {
				foo[j] = tmp[id[j]];
			}
			// printf("%s\n", foo + 1);
			if (str.find(hs(foo)) != str.end()) {
				good.push_back(i);
				fix[i] = 1;
				break;
			}
		} while (next_permutation(id + 1, id + n + 1));
		for (int j = 1; j < (1 << n); j++) {
			int fa = i;
			for (int k = 1; k <= n; k++) {
				if ((j >> (k - 1)) & 1) {
					fa -= get(fa, k) * pw[k - 1];
				}
			}
			son[fa].push_back(i);
			par[i].push_back(fa);
		}
	}
	if (good.empty()) {
		printf("impossible\n");
		return 0;
	}
	auto insert = [&](int id) {
		
		/*fprintf(stderr, "insert : ");
		for (int i = 1; i <= n; i++) cerr << get(id, i) << ' ';
		cerr << '\n';*/
		// fprintf(stderr, "dis = %.10Lf\n", dis[id]);
		for (auto i : par[id]) {
			sum[i] += dis[id];
			++cnt[i];
			dis[i] = (ld)sum[i] / cnt[i] + (ld)pw6[cnt0[i]] / cnt[i];
			// fprintf(stderr, "i = %d, dis[i] = %.10Lf, cnt[i] = %d\n", i, dis[i], cnt[i]);
			Q.push(make_pair(-dis[i], i));
		}
	};
	for (auto i : good) insert(i);
	while (!Q.empty()) {
		int u = Q.top().second; Q.pop();
		if (fix[u]) continue;
		fix[u] = 1;
		for (auto i : son[u]) {
			if (!fix[i]) {
				fix[i] = 1;
				dis[i] = dis[u];
				insert(i);
			}
		}
	}
	printf("%.10Lf\n", dis[0]);
	return 0;
}