int val;
int ledpin = 13;
void setup ()
{
 Serial.begin (9600);
 pinMode (ledpin, OUTPUT);
}
void loop ()
{
val = Serial.read ();
  digitalWrite (ledpin, HIGH);
  if (val == 'R'){
    digitalWrite (ledpin, LOW);
    delay (500);
    Serial.println ("Hello World!");
  }
}