other information related to the company's share options is as follows ( in millions ) : .
what was the average aggregate intrinsic value of stock options exercised from 2013 to 2015
unamortized deferred compensation expense , which includes both options and rsus , amounted to $ 378 million as of december 31 , 2015 , with a remaining weighted-average amortization period of approximately 2.1 years . employee share purchase plan united states the company has an employee share purchase plan that provides for the purchase of a maximum of 7.5 million shares of the company's ordinary shares by eligible u.s . employees . the company's ordinary shares were purchased at 6-month intervals at 85% ( 85 % ) of the lower of the fair market value of the ordinary shares on the first or last day of each 6-month period . in 2015 , 2014 , and 2013 , 411636 shares , 439000 shares and 556000 shares , respectively , were issued to employees under the plan . compensation expense recognized was $ 9 million in 2015 , $ 7 million in 2014 , and $ 6 million in 2013 . united kingdom the company also has an employee share purchase plan for eligible u.k . employees that provides for the purchase of shares after a 3-year period and that is similar to the u.s . plan previously described . three-year periods began in 2015 , 2014 , 2013 , allowing for the purchase of a maximum of 100000 , 300000 , and 350000 shares , respectively . in 2015 , 2014 , and 2013 , 2779 shares , 642 shares , and 172110 shares , respectively , were issued under the plan . compensation expense of $ 2 million was recognized in 2015 and 2014 , as compared to $ 1 million of compensation expense in 2013 . 12 . derivatives and hedging the company is exposed to market risks , including changes in foreign currency exchange rates and interest rates . to manage the risk related to these exposures , the company enters into various derivative instruments that reduce these risks by creating offsetting exposures . the company does not enter into derivative transactions for trading or speculative purposes . foreign exchange risk management the company is exposed to foreign exchange risk when it earns revenues , pays expenses , or enters into monetary intercompany transfers denominated in a currency that differs from its functional currency , or other transactions that are denominated in a currency other than its functional currency . the company uses foreign exchange derivatives , typically forward contracts , options and cross-currency swaps , to reduce its overall exposure to the effects of currency fluctuations on cash flows . these exposures are hedged , on average , for less than two years . these derivatives are accounted for as hedges , and changes in fair value are recorded each period in other comprehensive income ( loss ) in the consolidated statements of comprehensive income . the company also uses foreign exchange derivatives , typically forward contracts and options to economically hedge the currency exposure of the company's global liquidity profile , including monetary assets or liabilities that are denominated in a non-functional currency of an entity , typically on a rolling 30-day basis , but may be for up to one year in the future . these derivatives are not accounted for as hedges , and changes in fair value are recorded each period in other income in the consolidated statements of income. .