backlog backlog increased in 2015 compared to 2014 primarily due to higher orders on f-35 and c-130 programs . backlog decreased slightly in 2014 compared to 2013 primarily due to lower orders on f-16 and f-22 programs . trends we expect aeronautics 2019 2016 net sales to increase in the mid-single digit percentage range as compared to 2015 due to increased volume on the f-35 and c-130 programs , partially offset by decreased volume on the f-16 program . operating profit is also expected to increase in the low single-digit percentage range , driven by increased volume on the f-35 program offset by contract mix that results in a slight decrease in operating margins between years . information systems & global solutions our is&gs business segment provides advanced technology systems and expertise , integrated information technology solutions and management services across a broad spectrum of applications for civil , defense , intelligence and other government customers . is&gs 2019 technical services business provides a comprehensive portfolio of technical and sustainment services . is&gs has a portfolio of many smaller contracts as compared to our other business segments . is&gs has been impacted by the continued downturn in certain federal agencies 2019 information technology budgets and increased re-competition on existing contracts coupled with the fragmentation of large contracts into multiple smaller contracts that are awarded primarily on the basis of price . is&gs 2019 operating results included the following ( in millions ) : .

2015: net sales: 5596
operating profit: 508
operating margins: -9.1
backlog at year-end: 4800
2014: net sales: 5654
operating profit: 472
operating margins: -8.3
backlog at year-end: 6000
2013: net sales: 6115
operating profit: 498
operating margins: -8.1
backlog at year-end: 6300

2015 compared to 2014 is&gs 2019 net sales decreased $ 58 million , or 1% ( 1 % ) , in 2015 as compared to 2014 . the decrease was attributable to lower net sales of approximately $ 395 million as a result of key program completions , lower customer funding levels and increased competition , coupled with the fragmentation of existing large contracts into multiple smaller contracts that are awarded primarily on the basis of price when re-competed ( including cms-citic ) . these decreases were partially offset by higher net sales of approximately $ 230 million for businesses acquired in 2014 ; and approximately $ 110 million due to the start-up of new programs and growth in recently awarded programs . is&gs 2019 operating profit increased $ 36 million , or 8% ( 8 % ) , in 2015 as compared to 2014 . the increase was attributable to improved program performance and risk retirements , offset by decreased operating profit resulting from the activities mentioned above for net sales . adjustments not related to volume , including net profit booking rate adjustments and other matters , were approximately $ 70 million higher in 2015 compared to 2014 . 2014 compared to 2013 is&gs 2019 net sales decreased $ 461 million , or 8% ( 8 % ) , in 2014 as compared to 2013 . the decrease was primarily attributable to lower net sales of about $ 475 million due to the wind-down or completion of certain programs , driven by reductions in direct warfighter support ( including jieddo ) ; and approximately $ 320 million due to decreased volume in technical services programs reflecting market pressures . the decreases were offset by higher net sales of about $ 330 million due to the start-up of new programs , growth in recently awarded programs and integration of recently acquired companies . is&gs 2019 operating profit decreased $ 26 million , or 5% ( 5 % ) , in 2014 as compared to 2013 . the decrease was primarily attributable to the activities mentioned above for sales , partially offset by severance recoveries related to the restructuring announced in november 2013 of approximately $ 20 million in 2014 . adjustments not related to volume , including net profit booking rate adjustments , were comparable in 2014 and 2013. .

Questions:
what was the backlog at year-end in 2015?
and what was it in 2014?
what was, then, the total backlog at year-end for the two years?
including 2013, what then becomes that total for the three years?
and what is the backlog at year-end between those three years?

Answers:
4800
6000
10800
17100
5700