아두이노 우노 - 버튼 누르면 led에 순차적으로 점등

int led1 = 10;
int led2 = 11;
int led3 = 13;
int button = 9;
int b_num;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(led3, OUTPUT);
  pinMode(button, INPUT);
  digitalWrite(led1, true);
  b_num=1;
}

void loop() {
  // put your main code here, to run repeatedly:
  boolean b_state = digitalRead(button);

  if(b_state) {
    if(b_num == 1) {
      b_num = 2;
      Serial.println("2번째 LED");
      digitalWrite(led1, false);
      digitalWrite(led2, true);
      digitalWrite(led3, false);
    }
    else if(b_num == 2) {
      b_num = 3;
      Serial.println("3번째 LED");
      digitalWrite(led1, false);
      digitalWrite(led2, false);
      digitalWrite(led3, true);
    }
    else {
      b_num = 1;
      Serial.println("1번째 LED");
      digitalWrite(led1, true);
      digitalWrite(led2, false);
      digitalWrite(led3, false);
    }
  }
  delay(500);
}