#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
bool f(vector<string>s, vector<string>t) {
	rep(i, s.size())rep(j, s[i].size()) {
		if (s[i][j] == t[i][j])return false;
	}
	return true;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int h, w; cin >> h >> w;
	vector<string>s(h), t(h);
	rep(i, h)cin >> s[i];
	rep(i, h)cin >> t[i];
	bool chk1 = f(s, t);
	reverse(all(t));
	bool chk2 = f(s, t);
	double ans = 0;
	if (chk1 && !chk2)cout << "3.5317401904617327" << endl;
	else if (!chk1 && chk2)cout << "4.5317401904617327" << endl;
	else if (chk1 && chk2)cout << "2.64163256065515386629384" << endl;
	else cout << -1 << endl;
	return 0;
}