Armstrong Faber produces a standard number-two pencil called Ultra-Lite. The demand for Ultra-Lite has been fairly stable over the past ten years. On average, Armstrong Faber has sold 457,000 pencils each year. Furthermore, 90% of the time sales have been between 454,000 and 460,000 pencils. It is expected that the sales follow a normal distribution with a mean of 457,000 pencils. Estimate the standard deviation of this distribution.