#define ONBOARD_LED 13
#define EXTERNAL_LED 3
#define BUTTON  2
#define LED_OFF 0
#define LED_ON 1

int btn_counter = 0;

void setup() {
  // put your setup code here, to run once:
pinMode(ONBOARD_LED, OUTPUT);
pinMode(EXTERNAL_LED, OUTPUT);
pinMode(BUTTON, INPUT_PULLUP);
digitalWrite(ONBOARD_LED, HIGH); //5V
digitalWrite(EXTERNAL_LED, HIGH); //5V
}

void loop() {
  // put your main code here, to run repeatedly:
  /*
  digitalWrite(ONBOARD_LED, LOW);  //0V
  if(digitalRead(BUTTON) == HIGH) digitalWrite(EXTERNAL_LED, HIGH);  //0V
  delay(500);
  digitalWrite(ONBOARD_LED, HIGH); //5V 
  if(digitalRead(BUTTON) == HIGH) digitalWrite(EXTERNAL_LED, LOW);  //0V
  delay(500);
  */
   if(digitalRead(BUTTON) == LOW)
   {
    btn_counter++;
    while(digitalRead(BUTTON) == LOW);
   }
   if(btn_counter % 2 == 1) digitalWrite(EXTERNAL_LED, HIGH);
   else                     digitalWrite(EXTERNAL_LED, LOW);

}