chunk_size = 100_000
results = []

for chunk in pd.read_csv('large_sales_data.csv', chunksize=chunk_size):
    chunk['discounted_price'] = chunk['sales'] * (1 - chunk['discount'])
    results.append(chunk.groupby('region')['discounted_price'].sum())

final_result = pd.concat(results).groupby(level=0).sum()
print(final_result)