def unique_paths(m, n)
  dp = Array.new(m) { Array.new(n) }

  (0...m).each do |i|
    (0...n).each do |j|
      if i == 0 || j == 0
        dp[i][j] = 1
      else
        dp[i][j] = dp[i-1][j] + dp[i][j-1]
      end
    end
  end
  dp[m-1][n-1]
end