const int A=4;
const int B=2;
void setup()
{
  pinMode(A, OUTPUT);
  pinMode(B, OUTPUT);
}
void loop()
{
  digitalWrite(A,0);digitalWrite(B,0);
  delay(500);
  digitalWrite(A,0);digitalWrite(B,1);
  delay(500);
  digitalWrite(A,1);digitalWrite(B,0);
  delay(500);
  digitalWrite(A,1);digitalWrite(B,1);
  delay(500);
}