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 ) : .

1	2015	2014	2013
2	net sales	$ 9105	$ 9202	$ 9288
3	operating profit	1171	1187	1198
4	operating margins	12.9% ( 12.9 % )	12.9% ( 12.9 % )	12.9% ( 12.9 % )
5	backlog at year-end	$ 17400	$ 20300	$ 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. .

Previous questions in this conversation: what was the average backlog in 2015? what was it in 2014? what is the sum of those years?
Previous answers: 17400.0 20300.0 37700.0
Question: what was the average backlog in 2013?
Answer: 21400