// Define the area of interest (AOI) - Shandong, China
var aoi = ee.Geometry.Rectangle([115.5, 34.5, 122.5, 38.5]);

// Load Sentinel-2 data
var sentinel2 = ee.ImageCollection('COPERNICUS/S2')
  .filterBounds(aoi)
  .filterDate('2015-01-01', '2015-12-31')
  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 20))
  .select(['B4', 'B8']);

// Calculate NDVI
var addNDVI = function(image) {
  var ndvi = image.normalizedDifference(['B8', 'B4']).rename('NDVI');
  return image.addBands(ndvi);
};

var sentinel2WithNDVI = sentinel2.map(addNDVI).select('NDVI');

// Composite the collection to get median value
var sentinel2Composite = sentinel2WithNDVI.median().clip(aoi);

// Display the results
Map.centerObject(aoi, 8);
Map.addLayer(sentinel2Composite, {bands: ['NDVI'], min: 0, max: 1}, 'Sentinel-2 NDVI');

// Export the NDVI results to Google Drive
Export.image.toDrive({
  image: sentinel2Composite.select('NDVI'),
  description: 'Sentinel2_NDVI_Shandong_2015',
  scale: 1000,
  region: aoi,
  maxPixels: 1e13
});