void setup() {
  pinMode(0, OUTPUT); // Set pin '0' to OUTPUT mode
  pinMode(3, INPUT);  // Set pin '3' to INPUT mode
}

void loop() {
  int button = digitalRead(3); // Read pin 3 value and store it in 'button' variable

  if (button == HIGH) { // Compare 'button' variable value
    digitalWrite(0, HIGH); // Turn green LED on
  } else if (button == LOW) { // Compare 'button' variable value
    digitalWrite(0, LOW); // Turn green LED off
  }
}