/* -*- coding: utf-8 -*-
 *
 * 2871.cc:  No.2871 Universal Serial Bus - yukicoder
 */

#include<cstdio>
#include<cmath>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_H = 20;
const int MAX_W = 20;
const int CNT = 100;

/* typedef */

/* global variables */

char ss[MAX_H][MAX_W + 4], ts[MAX_H][MAX_W + 4];

/* subroutines */

bool check(int h, int w, bool rev) {
  for (int i = 0; i < h; i++)
    for (int j = 0; j < w; j++) {
      int ti = rev ? h - 1 - i : i;
      int tj = rev ? w - 1 - j : j;
      if (ss[i][j] == ts[ti][tj]) return false;
    }
  return true;
}

double calc(double p1, double p2) {
  double sum = 0.0, r = 1.0;
  for (int i = 1; i <= CNT; i++) {
    double pi = (i & 1) ? p1 : p2;
    double dr = r * (1.0 - pow(pi, -(i - 1)));
    sum += i * dr;
    r -= dr;
  }
  return sum;
}

/* main */

int main() {
  int h, w;
  scanf("%d%d", &h, &w);
  for (int i = 0; i < h; i++) scanf("%s", ss[i]);
  for (int i = 0; i < h; i++) scanf("%s", ts[i]);

  bool f0 = check(h, w, false), f1 = check(h, w, true);
  //printf("%d %d\n", f0, f1);

  double sum = calc(f0 ? 2 : 1, f1 ? 2 : 1);

  if (sum <= 0.0) puts("-1");
  else printf("%.12lf\n", sum);
  
  return 0;
}