int a=13;
int b=12;
int t=1000;

void setup() {
  // put your setup code here, to run once:
pinMode(a, OUTPUT);
pinMode(b, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(a, HIGH);
digitalWrite(b, LOW);
delay(t);
digitalWrite(a, LOW);
digitalWrite(b, HIGH);
delay(t);
}