int a, b, s;
void setup() {
  pinMode(13, INPUT_PULLUP);
  pinMode(12, INPUT_PULLUP);
  pinMode(4, OUTPUT);
  Serial.begin(9600);
}
void loop() {
  if (digitalRead(13) == 0) {
    a = millis();
    Serial.println(a);
  }
  if (digitalRead(12) == 0) {
    b = millis();
    Serial.println(b);
    s = b - a;
    digitalWrite(4, HIGH);
    delay(100);
    digitalWrite(4, LOW);
    delay(s);
    digitalWrite(4, HIGH);
    delay(100);
    digitalWrite(4, LOW);
  }
}