const int B = 12, L = 0;
bool s = 1;

void setup() {
  pinMode(B, INPUT_PULLUP);
  pinMode(L, OUTPUT);
  digitalWrite(L, s);
}

void loop() {
  if (!digitalRead(B)) {
    delay(50);
    if (!digitalRead(B)) {
      digitalWrite(L, s = !s);
      while (!digitalRead(B));
    }
  }
}