#define BUTTON_PIN 2
#define LED_PIN 4

int led_state=LOW;
int lastButtonstate=HIGH;
int buttonstate;

unsigned long lastDebouncetime = 0;
unsigned long debounceDelay= 50;

void setup()
{
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop()
{
  int reading=digitalRead(BUTTON_PIN);
  if(reading!=lastButtonstate)
  {
    lastDebouncetime=millis();
  }

  if((millis()-lastDebouncetime)>debounceDelay)
  {
    if(reading!=buttonstate)
    {
      buttonstate=reading;

      if(buttonstate==LOW)
      {
        led_state=!led_state;
        digitalWrite(LED_PIN, led_state);
      }
    }
  }
  lastButtonstate = reading;
}