#define  trigPin  13  // triger port in arduino
#define  echoPin  12  // echo port in arduino

void setup() {
  Serial.begin(9600);  // setting serial communication
  pinMode(trigPin, OUTPUT);  // Ultrasonic output mode(output by triggering)
  pinMode(echoPin, INPUT);  // Untrasonic input mode(input from reflection)
}

long microsecondsToCentimeters(long microseconds) {
  // speed of sound wave is 340 m/s, (340m/s) = (29 microsenconds / cm)
  return microseconds / 29 / 2;
}

void loop() {
  long duration,cm;

  digitalWrite(trigPin, LOW);
  delayMicroseconds(4);

  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);

  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH);

  cm = microsecondsToCentimeters(duration);

  Serial.write(cm); // ".print" is ASCII code but ".write"
  delay(200);
}