How to find the real distance between two ArUco markers.

edit

I got rvec and tvec from the estimating pose function. Then formed a 4x4 matrix with use of Rodrigues(rvec) and tvec

4x4 = [[ rvec rvec rvec tvecx]
       [ rvec rvec rvec tvecy]
       [ rvec rvec rvec tvecz]
       [ 0    0    0    1    ]]

Correct me if I am wrong and don't know how to proceed further.