import 'dart:io';

main() {
  print("What's your cat's age?");
  var age = int.parse(stdin.readLineSync()!);
    
    if (age == 1){
      print("15");  
    }
    if (age == 2){
      print("24");
    }
    if (age == 3){
      print(24 + 4*2);  
    }
    if (age > 3){
      print(24 + 4*(age - 2));
    }
    if (!(age == int)){ // this here should say that "if the variable age is not an integer"
    } else {
      print("Invalid age!");
    }
  
}

I have not learned yet how to express “if a variable does not hold an integer”

Thank you in advance