void setup() {
  Serial.begin(9600);
  randomSeed(analogRead(0));
  pinMode(9, INPUT_PULLUP);
  pinMode(7, INPUT_PULLUP);
  Serial.println("Welcome to reaction test! You need to press green button when the LED is on");
  Serial.println("Press red button to start the reaction test");
}
boolean button_7 = 0, system_on = 0, button_9 = 0;
unsigned long last_press = 0, reaction = 0, last_rand = 0;
void loop() {
  button_7 = !digitalRead(7);
  if (button_7 && millis() - last_press > 800){
    system_on = 1;
    last_press = millis();
    Serial.println("Reaction test started! Good luck!");
  }

  if (system_on){
  if (random(100) < 20 && millis() - last_rand > 4000){
    digitalWrite(8, HIGH);
    reaction = millis();
    last_rand = millis();
  }
  button_9 = !digitalRead(9);
  if (button_9 && millis() - last_press > 800){
  if (digitalRead(8)){
    reaction = millis() - reaction;
    Serial.print("Your reaction:");
    Serial.println(reaction);
    digitalWrite(8, LOW);
  } else {
    Serial.println("You are pressed button too early");
  }
  last_press = millis();
  }
}
}