Sam said to Joseph that ABCD is a square with side length 6. Circles X, Y, and Z are congruent circles with centers inside the square such that X is tangent to both sides AB and AD, Y is tangent to both sides AB and BC, and Z is tangent to side CD and both circles X and Y. The radii of circle X can be written as m - (n)^1/2 , when m and n are positive integers. What is m + n?