#define LED1 13
#define button1 2

volatile bool tooglestate = false;

void checksum() // ISR
{
  if(digitalRead(button1) == LOW)
  {
   // delay(200);
    tooglestate = !tooglestate;

    digitalWrite(LED1 , tooglestate);
  }
}

void setup() {
  // put your setup code here, to run once:
   
  pinMode(LED1 , OUTPUT);
  pinMode(button1 , INPUT_PULLUP);
  
  Serial.begin(9600);

  attachInterrupt(digitalPinToInterrupt(button1) , checksum , FALLING);
}

void loop() {
  // put your main code here, to run repeatedly :
  Serial.println("Delay started");
  delay(6000);
  Serial.println("Delay Finished");
  Serial.println("-----------------");

}