I had to reformat the text a little bit to make sure I knew what was going on, but here’s what I think you wanted to do.
(do
(print "enter a number: ")
(flush)
(def number1 (read-line))
(print "enter a second number: ")
(flush)
(def number2 (read-line))
(newline)
(println "[1] sum up")
(println "[2] substract")
(println "[3] multiplicate")
(println "[4] divide")
(print "input: ")
(flush)
(def operation (read-line))
(if (= operation "1")
(do
(print "solution:")
(println (+ (Integer/parseInt number1) (Integer/parseInt number2))))
(println "Something went wrong")))
Everything from read-line comes in as a string, you have to convert to the right type to call +