arduino按一下移一位怎麼寫呢?

int x1=13
int x2=12;
int x3=11;
int x4=10;
int x5=9;
int x6=8;
int a=1;
int buttonState=0;
void setup() {
pinMode(2, INPUT);
pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(11, OUTPUT);
pinMode(10, OUTPUT);
pinMode(9, OUTPUT);
pinMode(8, OUTPUT);
}
void loop() {
digitalWrite(13, HIGH);
digitalWrite(11, LOW);
delay(1000);
digitalWrite(11, HIGH);
digitalWrite(13, LOW);
delay(1000);
while(1)//
{
buttonState = digitalRead(2);
if (buttonState==HIGH)
{
a=a+1;
}
delay(1000);
if(a==1){
digitalWrite(12, HIGH);
digitalWrite(11, LOW);
}
if(a==2){
digitalWrite(13, HIGH);
digitalWrite(12, LOW);
}
}
}
現在只做按一下按鈕+1下燈號,但是我燈號停止時未按鈕都是11,當我另外再設一按鈕當-1,那我按二下到13時,此時再按另一按鈕-1,不就只能設11暗10亮,這樣13亮不就瞬間跳到10亮,我要怎麼設讓哪一個燈亮是變數,而不是定數,這樣不管怎麼按,燈都只是一個個往上和往下