2014 compared to 2013 mst 2019s net sales decreased $ 305 million , or 3% ( 3 % ) , in 2014 as compared to 2013 . net sales decreased by approximately $ 305 million due to the wind-down or completion of certain c4isr programs ( primarily ptds ) ; about $ 85 million for undersea systems programs due to decreased volume and deliveries ; and about $ 55 million related to the settlements of contract cost matters on certain programs in 2013 that were not repeated in 2014 ( including a portion of the terminated presidential helicopter program ) . the decreases were partially offset by higher net sales of approximately $ 80 million for integrated warfare systems and sensors programs due to increased volume ( primarily space fence ) ; and approximately $ 40 million for training and logistics solutions programs due to increased deliveries ( primarily close combat tactical trainer ) . mst 2019s operating profit decreased $ 129 million , or 12% ( 12 % ) , in 2014 as compared to 2013 . the decrease was primarily attributable to lower operating profit of approximately $ 120 million related to the settlements of contract cost matters on certain programs in 2013 that were not repeated in 2014 ( including a portion of the terminated presidential helicopter program ) ; approximately $ 55 million due to the reasons described above for lower c4isr program sales , as well as performance matters on an international program ; and approximately $ 45 million due to higher reserves recorded on certain training and logistics solutions programs . the decreases were partially offset by higher operating profit of approximately $ 45 million for performance matters and reserves recorded in 2013 that were not repeated in 2014 ; and about $ 60 million for various programs due to increased risk retirements ( including mh-60 and radar surveillance programs ) . adjustments not related to volume , including net profit booking rate adjustments and other matters , were approximately $ 85 million lower for 2014 compared to 2013 . backlog backlog increased in 2015 compared to 2014 primarily due to the addition of sikorsky backlog , as well as higher orders on new program starts ( such as australian defence force pilot training system ) . backlog increased in 2014 compared to 2013 primarily due to higher orders on new program starts ( such as space fence ) . trends we expect mst 2019s 2016 net sales to increase in the mid-double digit percentage range compared to 2015 net sales due to the inclusion of sikorsky programs for a full year , partially offset by a decline in volume due to the wind-down or completion of certain programs . operating profit is expected to be equivalent to 2015 on higher volume , and operating margin is expected to decline due to costs associated with the sikorsky acquisition , including the impact of purchase accounting adjustments , integration costs and inherited restructuring costs associated with actions committed to by sikorsky prior to acquisition . space systems our space systems business segment is engaged in the research and development , design , engineering and production of satellites , strategic and defensive missile systems and space transportation systems . space systems provides network-enabled situational awareness and integrates complex global systems to help our customers gather , analyze , and securely distribute critical intelligence data . space systems is also responsible for various classified systems and services in support of vital national security systems . space systems 2019 major programs include the trident ii d5 fleet ballistic missile ( fbm ) , orion , space based infrared system ( sbirs ) , aehf , gps-iii , geostationary operational environmental satellite r-series ( goes-r ) , and muos . operating profit for our space systems business segment includes our share of earnings for our investment in ula , which provides expendable launch services to the u.s . government . space systems 2019 operating results included the following ( in millions ) : .

2015: net sales: 9105
operating profit: 1171
operating margins: -12.9
backlog at year-end: 17400
2014: net sales: 9202
operating profit: 1187
operating margins: -12.9
backlog at year-end: 20300
2013: net sales: 9288
operating profit: 1198
operating margins: -12.9
backlog at year-end: 21400

2015 compared to 2014 space systems 2019 net sales in 2015 decreased $ 97 million , or 1% ( 1 % ) , compared to 2014 . the decrease was attributable to approximately $ 335 million lower net sales for government satellite programs due to decreased volume ( primarily aehf ) and the wind-down or completion of mission solutions programs ; and approximately $ 55 million for strategic missile and defense systems due to lower volume . these decreases were partially offset by higher net sales of approximately $ 235 million for businesses acquired in 2014 ; and approximately $ 75 million for the orion program due to increased volume. .

Questions:
what was the average backlog in 2015?
what was it in 2014?
what is the sum of those years?
what was the average backlog in 2013?
what is the total sum for all 3 years?
what is the average per year?

Answers:
17400
20300
37700
21400
59100
19700