unsigned long startTime;

void setup() {
  pinMode(A0, OUTPUT);
  pinMode(A8, INPUT_PULLUP);
  Serial.begin(9600);
  Serial.println("Hello, reation timer game");
  delay(random(1000, 4000));
  digitalWrite(A0, HIGH);
  startTime = millis();
}

void loop() {
  if (digitalRead(A8) == LOW) {
    unsigned long reaction = millis() - startTime;
    Serial.print("Reaction Time: ");
    Serial.println(reaction);
    digitalWrite(A0, LOW);
    while(1);
  }
}