#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
#define ll long long
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
using mint = modint998244353;
#define ull unsigned long long
random_device rnd;
mt19937 mt(rnd());
int RandInt(int a, int b) {
    return a + mt() % (b - a + 1);
}

int masu_a[22][22];
int masu_b[22][22];
int main(){
    int h,w;
    cin>>h>>w;
    vector<string> s(h);
    rep(i,h) cin>>s[i];
    vector<string>t(h);
    rep(i,h) cin>>t[i];
    double ans=0;
    int cnt=0;
    rep(i,h)rep(j,w){
        if(s[i][j]!=t[i][j]) cnt++;
    }
    if(cnt!=h*w){
        cout<<-1<<endl;
        return 0;
    }
    double bai=1.0;
    double tmp=0.0,nokori=1.0;
    for(int i=1;i<=h*w;i++){
        tmp=(1.0-1.0/pow(2,i-1))*nokori;
        
        //cerr<<tmp<<" "<<nokori<<endl;
        if(i%2==1){
            ans+=(double)i*tmp;
            //cerr<<"i:"<<i<<" "<<tmp<<endl;
            nokori-=tmp;
        }
        
        bai*=2.0;
    }
    cout<<fixed<<setprecision(10)<<ans<<endl;
    return 0;
}