Please help with simple arms macro

Hi! I have tried almost all arms macros on the site, but no one work as i would like. Can you please help me to create a simple priority macro? I would like just to use 2 abilities in macro. Mortal Strike and Slam, and i would like, that the macro should always cast Ms when avaible, and just than Slam.I tried with priority:
“/cast Mortal Strike”,
“/cast Slam”,
Or
“/castsequence Mortal Strike”,
“/cast Mortal Strike”,
“/cast Slam”,
The error is, that sometimes when Ms proc or is not on cd, it cast 2-3 Slams before casting Ms. Do you have any ideas,macro or solutions for my problem? I use the macro on a simple mouse button!Thx in advance