/*
If I send 42 with a new line I get:

Available before: 3
Number: 42
Available after: 1
Leftover bytes in serial buffer: 0A

*/
void setup() {
  Serial.begin(115200);
}

void loop() {
  if (Serial.available()) {
    Serial.print("Available before: ");
    Serial.println(Serial.available());
    Serial.print("Number: ");
    Serial.println(Serial.parseInt());
    Serial.print("Available after: ");
    Serial.println(Serial.available());
    if (Serial.available()) {
      Serial.print("Leftover bytes in serial buffer:");
      if (Serial.available()) {
        Serial.print(" ");
        byte b = Serial.read();
        if (b < 0x0F)
          Serial.print("0");
        Serial.print(b, HEX);
      }
      Serial.println();
    }
    Serial.println();
  }
}