note 11 2013 stock-based compensation during 2014 , 2013 and 2012 , we recorded non-cash stock-based compensation expense totaling $ 164 million , $ 189 million and $ 167 million , which is included as a component of other unallocated , net on our statements of earnings . the net impact to earnings for the respective years was $ 107 million , $ 122 million and $ 108 million . as of december 31 , 2014 , we had $ 91 million of unrecognized compensation cost related to nonvested awards , which is expected to be recognized over a weighted average period of 1.6 years . we received cash from the exercise of stock options totaling $ 308 million , $ 827 million and $ 440 million during 2014 , 2013 and 2012 . in addition , our income tax liabilities for 2014 , 2013 and 2012 were reduced by $ 215 million , $ 158 million , $ 96 million due to recognized tax benefits on stock-based compensation arrangements . stock-based compensation plans under plans approved by our stockholders , we are authorized to grant key employees stock-based incentive awards , including options to purchase common stock , stock appreciation rights , restricted stock units ( rsus ) , performance stock units ( psus ) or other stock units . the exercise price of options to purchase common stock may not be less than the fair market value of our stock on the date of grant . no award of stock options may become fully vested prior to the third anniversary of the grant and no portion of a stock option grant may become vested in less than one year . the minimum vesting period for restricted stock or stock units payable in stock is three years . award agreements may provide for shorter or pro-rated vesting periods or vesting following termination of employment in the case of death , disability , divestiture , retirement , change of control or layoff . the maximum term of a stock option or any other award is 10 years . at december 31 , 2014 , inclusive of the shares reserved for outstanding stock options , rsus and psus , we had 19 million shares reserved for issuance under the plans . at december 31 , 2014 , 7.8 million of the shares reserved for issuance remained available for grant under our stock-based compensation plans . we issue new shares upon the exercise of stock options or when restrictions on rsus and psus have been satisfied . the following table summarizes activity related to nonvested rsus during 2014 : number of rsus ( in thousands ) weighted average grant-date fair value per share .
what was the percentage change in non-cash stock-based compensation expense from 2013 to 2014?
rsus are valued based on the fair value of our common stock on the date of grant . employees who are granted rsus receive the right to receive shares of stock after completion of the vesting period ; however , the shares are not issued and the employees cannot sell or transfer shares prior to vesting and have no voting rights until the rsus vest , generally three years from the date of the award . employees who are granted rsus receive dividend-equivalent cash payments only upon vesting . for these rsu awards , the grant-date fair value is equal to the closing market price of our common stock on the date of grant less a discount to reflect the delay in payment of dividend-equivalent cash payments . we recognize the grant-date fair value of rsus , less estimated forfeitures , as compensation expense ratably over the requisite service period , which beginning with the rsus granted in 2013 is shorter than the vesting period if the employee is retirement eligible on the date of grant or will become retirement eligible before the end of the vesting period. .