generally your blocks look fine... but how does your start value look like?
to find it out, use Do it to debug your blocks, especially use Do it to get the value of the get start value block, see also tip 4 here App Inventor: How to Learn | Pura Vida Apps
see also Live Development, Testing, and Debugging Tools
see also tip 1 here and switch screens correctly...
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by
Taifun.