aeronautics our aeronautics business segment is engaged in the research , design , development , manufacture , integration , sustainment , support and upgrade of advanced military aircraft , including combat and air mobility aircraft , unmanned air vehicles and related technologies . aeronautics 2019 major programs include the f-35 lightning ii joint strike fighter , c-130 hercules , f-16 fighting falcon , c-5m super galaxy and f-22 raptor . aeronautics 2019 operating results included the following ( in millions ) : .
what was the average backlog at year-end in millions from 2013 to 2015?
2015 compared to 2014 aeronautics 2019 net sales in 2015 increased $ 650 million , or 4% ( 4 % ) , compared to 2014 . the increase was attributable to higher net sales of approximately $ 1.4 billion for f-35 production contracts due to increased volume on aircraft production and sustainment activities ; and approximately $ 150 million for the c-5 program due to increased deliveries ( nine aircraft delivered in 2015 compared to seven delivered in 2014 ) . the increases were partially offset by lower net sales of approximately $ 350 million for the c-130 program due to fewer aircraft deliveries ( 21 aircraft delivered in 2015 , compared to 24 delivered in 2014 ) , lower sustainment activities and aircraft contract mix ; approximately $ 200 million due to decreased volume and lower risk retirements on various programs ; approximately $ 195 million for the f-16 program due to fewer deliveries ( 11 aircraft delivered in 2015 , compared to 17 delivered in 2014 ) ; and approximately $ 190 million for the f-22 program as a result of decreased sustainment activities . aeronautics 2019 operating profit in 2015 increased $ 32 million , or 2% ( 2 % ) , compared to 2014 . operating profit increased by approximately $ 240 million for f-35 production contracts due to increased volume and risk retirements ; and approximately $ 40 million for the c-5 program due to increased risk retirements . these increases were offset by lower operating profit of approximately $ 90 million for the f-22 program due to lower risk retirements ; approximately $ 70 million for the c-130 program as a result of the reasons stated above for lower net sales ; and approximately $ 80 million due to decreased volume and risk retirements on various programs . adjustments not related to volume , including net profit booking rate adjustments and other matters , were approximately $ 100 million higher in 2015 compared to 2014 . 2014 compared to 2013 aeronautics 2019 net sales increased $ 797 million , or 6% ( 6 % ) , in 2014 as compared to 2013 . the increase was primarily attributable to higher net sales of approximately $ 790 million for f-35 production contracts due to increased volume and sustainment activities ; about $ 55 million for the f-16 program due to increased deliveries ( 17 aircraft delivered in 2014 compared to 13 delivered in 2013 ) partially offset by contract mix ; and approximately $ 45 million for the f-22 program due to increased risk retirements . the increases were partially offset by lower net sales of approximately $ 55 million for the f-35 development contract due to decreased volume , partially offset by the absence in 2014 of the downward revision to the profit booking rate that occurred in 2013 ; and about $ 40 million for the c-130 program due to fewer deliveries ( 24 aircraft delivered in 2014 compared to 25 delivered in 2013 ) and decreased sustainment activities , partially offset by contract mix . aeronautics 2019 operating profit increased $ 37 million , or 2% ( 2 % ) , in 2014 as compared to 2013 . the increase was primarily attributable to higher operating profit of approximately $ 85 million for the f-35 development contract due to the absence in 2014 of the downward revision to the profit booking rate that occurred in 2013 ; about $ 75 million for the f-22 program due to increased risk retirements ; approximately $ 50 million for the c-130 program due to increased risk retirements and contract mix , partially offset by fewer deliveries ; and about $ 25 million for the c-5 program due to the absence in 2014 of the downward revisions to the profit booking rate that occurred in 2013 . the increases were partially offset by lower operating profit of approximately $ 130 million for the f-16 program due to decreased risk retirements , partially offset by increased deliveries ; and about $ 70 million for sustainment activities due to decreased risk retirements and volume . operating profit was comparable for f-35 production contracts as higher volume was offset by lower risk retirements . adjustments not related to volume , including net profit booking rate adjustments and other matters , were approximately $ 105 million lower for 2014 compared to 2013. .