void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial.println("Enter a number greater than 0");
  Serial.setTimeout(10); //this is the time bit takes for the serial communicator to 
  // time out. reducing this time means some things can be recieved quicker, but if it takes longer
  // to run the code that the time out time is, then nothing will be printed. 
 
}

void loop() {
  // put your main code here, to run repeatedly:
  if (Serial.available() > 0){
    int var = Serial.parseInt(); //this passes the value as an integer. Using parseFloat will pass a decimal number
    if (var > 0){
      Serial.println(var);
    }
    else{
      Serial.println("Enter a valid number");
    }
  } // to send a string, you have to use readString. The variable var must be changed to a String variable.
}

// the reason why you get a zero after every number is becasue new line ending is enabled
// this is \n. - on IDE you can change the communication to have no ending