void setup() {
  pinMode(PA3, OUTPUT); //Button Pin
  pinMode(PA7, OUTPUT); //LED pin
}

void loop() {
  if (digitalRead(PA3))
  {
    digitalWrite(LED_BUILTIN, HIGH);
    delay(500);
    digitalWrite(LED_BUILTIN, LOW);
    delay(500);
  }
  else
  {
    digitalWrite(LED_BUILTIN, LOW);
  }
}