$4.5 million for the year ended December 31, 2013.

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Suboptimal Exercise Factor: Our computation of the suboptimal exercise factor is based on historical option exercise behavior and the terms and vesting periods of the options granted and is determined for both executives and non-executives. An increase in the suboptimal exercise factor of 10% would increase the total stock-based compensation expense by approximately $2.4 million for the year ended December 31, 2013.