#include <Servo.h>
int echoPin =2;
int trigPin =3;
Servo myServo;
void setup() {
Serial.begin(9600);
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
myServo.attach(5);
}

void loop() {
  myServo.write(0);
int duration, cm;
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
duration = pulseIn(echoPin,HIGH);
cm = duration /58;
if ( cm <=20)
{
  myServo.write(90);
  delay(500);
}
else 
myServo.write(0);
delay(500);
Serial.print(cm);
Serial.println("cm");
delay(300);
}