FAST_LED_2.1_quick and dirty strobe

#include "FastLED.h"

// How many leds in your strip?
#define NUM_LEDS 300

// For led chips like Neopixels, which have a data line, ground, and power, you just
// need to define DATA_PIN.  For led chipsets that are SPI based (four wires - data, clock,
// ground, and power), like the LPD8806 define both DATA_PIN and CLOCK_PIN
#define DATA_PIN 6
#define CLOCK_PIN 13


CRGB leds[NUM_LEDS];

long sthalf = (NUM_LEDS/2);
long sthalf2 = (NUM_LEDS);   //THIS IS WHERE I'M HAVING TROUBLE

void setup() {
  // Uncomment/edit one of the following lines for your leds arrangement.
  // FastLED.addLeds<TM1803, DATA_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<TM1804, DATA_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<TM1809, DATA_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<WS2811, DATA_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<WS2812, DATA_PIN, RGB>(leds, NUM_LEDS);
  FastLED.addLeds<WS2812B, DATA_PIN, GRB>(leds, NUM_LEDS);
  //  FastLED.addLeds<NEOPIXEL, DATA_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<UCS1903, DATA_PIN, RGB>(leds, NUM_LEDS);

  // FastLED.addLeds<WS2801, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<SM16716, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<LPD8806, RGB>(leds, NUM_LEDS);

  // FastLED.addLeds<WS2801, DATA_PIN, CLOCK_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<SM16716, DATA_PIN, CLOCK_PIN, RGB>(leds, NUM_LEDS);
  // FastLED.addLeds<LPD8806, DATA_PIN, CLOCK_PIN, RGB>(leds, NUM_LEDS);
}

void loop(){
  fill_solid(leds, sthalf, CRGB::White);
  FastLED.show();
  delay(20);
  fill_solid(leds, NUM_LEDS, CRGB::Black);

  FastLED.show();
  delay(1000);

  fill_solid(leds, sthalf2, CRGB::White);
  FastLED.show();
  delay(20);
  fill_solid(leds, sthalf2, CRGB::Black);

  FastLED.show();
  delay(1000);
}