volatile int echo_start, echo_end, d, duration;


void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);

EICRA=EICRA&0b00000000;
EICRA=EICRA|0B00000001;
SREG=0X80;
EIMSK=EIMSK|0b00000001;
pinMode(3,OUTPUT);
pinMode(2,INPUT);
}

ISR(INT0_vect){
  if(digitalRead(2)==1){
    echo_start=micros();
  }
  echo_end=micros();
  duration=(echo_end-echo_start);
}
void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(3, HIGH);
  delay(10);
  digitalWrite(3, LOW);
  d=(duration*0.001715);
  Serial.println(d);
  delay(500);
}