const int rin = 2;    

//void prerus();

void setup() 
{
  Serial.begin(9600);
  Serial.print("setup");
  Serial.println();
  pinMode(rin, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), prerus, FALLING);
}

void loop() 
{
Serial.print("loop");
Serial.println();
delay(1000);
}


void prerus() 
{
  Serial.print("interrupt");
  Serial.println();
}