h, w = read_line.split.map(&.to_i)
s = Array.new(h) { read_line.chars.map { |ch| ch == '#' } }
t = Array.new(h) { read_line.chars.map { |ch| ch == '#' } }
ok0 = h.times.all? { |i| w.times.all? { |j| s[i][j] != t[i][j] } }
s.reverse!
s.map! { |r| r.reverse }
ok1 = h.times.all? { |i| w.times.all? { |j| s[i][j] != t[i][j] } }
if !ok0 && !ok1
  puts -1
  exit
end

ans = 0.0
fail = 1.0
prob = 1.0
100000.times do |i|
  if [ok0, ok1][i % 2]
    ans += (i + 1) * (1.0 - fail) * prob
    prob -= prob * (1.0 - fail)
  end
  fail *= 0.5
end

printf("%.9f\n", ans)