If Else question

[InterestedNewbie]

The following probably needs and If Else statement but I'm not sure how to implement it (and where).... 

The script is closing App1 and then it is starting App2. 
If App2 is not found on the user's computer App1 will be restarted.

Code:

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ShutDownProcess("app1")
1
run "$program files$\Apps\app2.exe"
1
OnScreenDisplay "app2 is not installed, launching app1" 2.5
3
run "$program files$\Apps\app1.exe"