each clearing firm is required to deposit and maintain balances in the form of cash , u.s . government securities , certain foreign government securities , bank letters of credit or other approved investments to satisfy performance bond and guaranty fund requirements . all non-cash deposits are marked-to-market and haircut on a daily basis . securities deposited by the clearing firms are not reflected in the consolidated financial statements and the clearing house does not earn any interest on these deposits . these balances may fluctuate significantly over time due to investment choices available to clearing firms and changes in the amount of contributions required . in addition , the rules and regulations of cbot require that collateral be provided for delivery of physical commodities , maintenance of capital requirements and deposits on pending arbitration matters . to satisfy these requirements , clearing firms that have accounts that trade certain cbot products have deposited cash , u.s . treasury securities or letters of credit . the clearing house marks-to-market open positions at least once a day ( twice a day for futures and options contracts ) , and require payment from clearing firms whose positions have lost value and make payments to clearing firms whose positions have gained value . the clearing house has the capability to mark-to-market more frequently as market conditions warrant . under the extremely unlikely scenario of simultaneous default by every clearing firm who has open positions with unrealized losses , the maximum exposure related to positions other than credit default and interest rate swap contracts would be one half day of changes in fair value of all open positions , before considering the clearing houses 2019 ability to access defaulting clearing firms 2019 collateral deposits . for cleared credit default swap and interest rate swap contracts , the maximum exposure related to cme 2019s guarantee would be one full day of changes in fair value of all open positions , before considering cme 2019s ability to access defaulting clearing firms 2019 collateral . during 2017 , the clearing house transferred an average of approximately $ 2.4 billion a day through the clearing system for settlement from clearing firms whose positions had lost value to clearing firms whose positions had gained value . the clearing house reduces the guarantee exposure through initial and maintenance performance bond requirements and mandatory guaranty fund contributions . the company believes that the guarantee liability is immaterial and therefore has not recorded any liability at december 31 , 2017 . at december 31 , 2016 , performance bond and guaranty fund contribution assets on the consolidated balance sheets included cash as well as u.s . treasury and u.s . government agency securities with maturity dates of 90 days or less . the u.s . treasury and u.s . government agency securities were purchased by cme , at its discretion , using cash collateral . the benefits , including interest earned , and risks of ownership accrue to cme . interest earned is included in investment income on the consolidated statements of income . there were no u.s . treasury and u.s . government agency securities held at december 31 , 2017 . the amortized cost and fair value of these securities at december 31 , 2016 were as follows : ( in millions ) amortized .

2016 amortizedcost: u.s . treasury securities: 5548.9
u.s . government agency securities: 1228.3
2016 fairvalue: u.s . treasury securities: 5549
u.s . government agency securities: 1228.3

cme has been designated as a systemically important financial market utility by the financial stability oversight council and maintains a cash account at the federal reserve bank of chicago . at december 31 , 2017 and december 31 , 2016 , cme maintained $ 34.2 billion and $ 6.2 billion , respectively , within the cash account at the federal reserve bank of chicago . clearing firms , at their option , may instruct cme to deposit the cash held by cme into one of the ief programs . the total principal in the ief programs was $ 1.1 billion at december 31 , 2017 and $ 6.8 billion at december 31 .

Questions:
what was the balance within the cash account at the federal reserve bank of chicago at the end of 2017?
what was it at the end of 2016?
what is the sum value?
what is the sum divided by 2?

Answers:
34.2
6.2
40.4
20.2