const int LED_PIN = 4;
const int BTN_PIN = 5;

void setup() {
  Serial.begin(115200);
  pinMode( LED_PIN, OUTPUT );
  // Use the internal pullup resistor on GPIO pin.
  pinMode( BTN_PIN, INPUT_PULLUP );
}

int saved_value = HIGH; // Used to save the last changed value.

void loop() {
  // Read the input value from the button pin.
  int value = digitalRead( BTN_PIN );
  if ( saved_value != value ) {
    // Show the input value.
    Serial.printf( "Input value changed: %d\n", value );
    // Save the new input value.
    saved_value = value;
  }
  // Write the inverted input value to the output LED pin.
  digitalWrite( LED_PIN, !value );
  delay(10);
}