kotlin reject negative, string and float input

Question

I'm not allowed to change the toIntOrNull() and I need to reject every string, float, and negative input. I tried to do it with n is String but I just encountered and error. Is there any other way to do it?

Here's my code:

fun main(){
    print("Enter number of numbers from Fibonacci: ")
    var n: Int? = readLine()?.toIntOrNull()
    
    //print("Input is Invalid!")
    var total = fib(n)
    print("Total $total")
}

An example of what should appear when a string, double or negative is entered:

Enter number of numbers from Fibonacci: one
Input is Invalid!