#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ull=unsigned long long;
using pll=pair<ll,ll>;
using tll=tuple<ll,ll,ll>;
using ld=long double;
const ll INF=(1ll<<60);
#define rep(i,n) for (ll i=0;i<(ll)(n);i++)
#define replr(i,l,r) for (ll i=(ll)(l);i<(ll)(r);i++)
#define all(v) v.begin(),v.end()
#define len(v) ((ll)v.size())
template<class T> inline bool chmin(T &a,T b){
    if(a>b){
        a=b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T &a,T b){
    if(a<b){
        a=b;
        return true;
    }
    return false;
}
namespace timer{
    auto start=chrono::system_clock::now();
    void snap(){
        start=chrono::system_clock::now();
    }
    int get_ms(){
        auto now=chrono::system_clock::now();
        int ms=chrono::duration_cast<chrono::milliseconds>(now-start).count();
        return ms;
    }
}
mt19937 mt;
inline double rand_double(){  //[0.0,1.0]
    return (double)mt()/(double)mt19937::max();
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll h,w;
    cin >> h >> w;
    vector<string> s(h),t(h);
    rep(i,h) cin >> s[i];
    rep(i,h) cin >> t[i];
    vector<ll> v(2,true);
    rep(i,h){
        rep(j,w){
            if(s[i][j]==t[i][j]) v[0]=false;
        }
    }
    reverse(all(s));
    rep(i,h) reverse(all(s[i]));
    rep(i,h){
        rep(j,w){
            if(s[i][j]==t[i][j]) v[1]=false;
        }
    }
    if(!v[0]&&!v[1]){
        cout << "-1\n";
        return 0;
    }
    ll cnt=0,sum=0;
    while(timer::get_ms()<1900){
        ll now=1;
        while(true){
            if(v[1-(now&1)]&&rand_double()<1-pow(2,-(now-1))) break;
            now++;
        }
        cnt++;
        sum+=now;
    }
    cout << fixed << setprecision(20) << (ld)sum/(ld)cnt << '\n';
}