void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}
void loop() {
 int A=random(500,5000);
   Serial.println(A);
  int B=random(500,5000);
   Serial.println(B);
  digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(A);                       
  digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW
  delay(B);  

}