int pir_ip = 12;
int pir_op = 10;  
int valip = 0; // variable for reading the pin status
int valop = 0;
int counter = 0;

void setup() {
pinMode(pir_ip, INPUT); // declare LED as output
pinMode(pir_op, INPUT); // declare sensor as input
 Serial.begin(9600);
}

void loop()
{valip = digitalRead(pir_ip);
 valop = digitalRead(pir_op);
  counter = counter + valip - valop;
  Serial.print(counter,DEC);
  delay(3000);
}