Japt, 6 bytes

@øX}f1

Try it here

Japt, 6 bytes

@øX}f1

Try it

@øX}f1     :Implicit input of array U
@          :Function taking an integer X as argument
 øX        :  Does U contain X
   }       :End function
    f1     :Get the first integer >=1 that returns false