### Matrix rotation error and how pull out a single element

Hi,
I have problem with matrix rotation. I am using the module Aruco to receive marker position. Then I use the function Rodrigues () to receive a rotation matrix. I would like to pull out of the matrix as a single element needs to calculate the orientation of the marker. But all the time I error.
The following code:

```
cv::Mat rvecs, tvecs;
// detect markers and estimate pose
aruco::detectMarkers(image, dictionary, corners, ids, detectorParams, rejected);
if (estimatePose && ids.size() > 0)
aruco::estimatePoseSingleMarkers(corners, markerLength, camMatrix, distCoeffs, rvecs, tvecs);
double currentTime = ((double)getTickCount() - tick) / getTickFrequency();
Mat R = Mat::zeros(3, 3, CV_64F);
Rodrigues(rvecs, R);
```

Here is the formula:

```
Mat Thz = (atan2 (R (3), R (0))) * (180 / M_PI)
```

But the problem ~~lies ~~is in the variables.

When ~~you ~~I'm use `R.at <double> (1, 1)`

displays a single element matrix. For example, when I give `Mat thz = atan2 (R.at <double> (1, 1), R.at <double> (1, 2));`

,

displays error `C2440: 'initializing': can not convert from 'double' this' cv :: Mat`

How do I convert the matrix R so I can use them with designs and functions atan2?