Table of Contents ITEM 2.
PROPERTIES Flight Equipment and Fleet Renewal As of December 31, 2016, American operated a mainline fleet of 930 aircraft.
In 2016, we continued our extensive fleet renewal program, which has provided us with the youngest fleet of the major U. S.  network carriers.
During 2016, American took delivery of 55 new mainline aircraft and retired 71 aircraft.
We are supported by our wholly-owned and third-party regional carriers that fly under capacity purchase agreements operating as American Eagle.
As of December 31, 2016, American Eagle operated 606 regional aircraft.
During 2016, we increased our regional fleet by 61 regional aircraft, we removed and placed in temporary storage one Embraer ERJ 140 aircraft and retired 41 other regional aircraft.
Mainline As of December 31, 2016, American’s mainline fleet consisted of the following aircraft:
||Average Seating  Capacity|Average  Age  (Years)|Owned|Leased|Total|
|Airbus A319|128|12.8|19|106|125|
|Airbus A320|150|15.5|10|41|51|
|Airbus A321|178|4.9|153|46|199|
|AirbusA330-200|258|5.0|15|—|15|
|AirbusA330-300|291|16.4|4|5|9|
|Boeing737-800|160|7.7|123|161|284|
|Boeing757-200|179|17.9|39|12|51|
|Boeing767-300ER|211|19.5|28|3|31|
|Boeing777-200ER|263|16.0|44|3|47|
|Boeing777-300ER|310|2.8|18|2|20|
|Boeing787-8|226|1.3|17|—|17|
|Boeing787-9|285|0.2|4|—|4|
|Embraer 190|99|9.2|20|—|20|
|McDonnell DouglasMD-80|140|22.0|25|32|57|
|Total||10.3|519|411|930|

CF INDUSTRIES HOLDINGS, INC.  123 Changes in common shares outstanding are as follows:
||Year ended December 31,|
||2016|2015|2014|
|Beginning balance|233,081,556|241,673,050|279,240,970|
|Exercise of stock options|17,600|274,705|942,560|
|Issuance of restricted stock-1|44,941|40,673|20,875|
|Forfeitures of restricted stock|-10,000|—|-65,680|
|Purchase of treasury shares-2|-19,928|-8,906,872|-38,465,675|
|Ending balance|233,114,169|233,081,556|241,673,050|

(1) Includes shares issued from treasury.
(2) Includes shares withheld to pay employee tax obligations upon the vesting of restricted stock.
Preferred Stock CF Holdings is authorized to issue 50 million shares of $0.01 par value preferred stock.
Our Second Amended and Restated Certificate of Incorporation, as amended, authorizes the Board, without any further stockholder action or approval, to issue these shares in one or more classes or series, and (except in the case of our Series A Junior Participating Preferred Stock, 500,000 shares of which are authorized and the terms of which were specified in the original certificate of incorporation of CF Holdings) to fix the rights, preferences and privileges of the shares of each wholly unissued class or series and any of its qualifications, limitations or restrictions.
In connection with the Plan (as defined below), 500,000 shares of preferred stock have been designated as Series B Junior Participating Preferred Stock.
The Series A Junior Participating Preferred Stock had been established in CF Holdings’ original certificate of incorporation in connection with our former stockholder rights plan that expired in 2015.
No shares of preferred stock have been issued.
Tax Benefits Preservation Plan On September 6, 2016, CF Holdings entered into a Tax Benefits Preservation Plan (the Plan) with Computershare Trust Company, N. A. , as rights agent.
The Plan is intended to help protect our tax net operating losses and certain other tax assets (the Tax Benefits) by deterring any person from becoming a "5-percent shareholder" (as defined in Section 382 of the Internal Revenue Code of 1986, as amended) (a 5% Shareholder).
Under the Plan, each share of common stock has attached to it one right.
Each right entitles the holder to purchase one one-thousandth of a share of our preferred stock designated as Series B Junior Participating Preferred Stock at a purchase price of $100, subject to adjustment.
Rights will only be exercisable under the limited circumstances specified in the Plan when there has been a distribution of the rights and such rights are no longer redeemable by CF Holdings.
A distribution of the rights would occur upon the earlier of (i) 10 business days following a public announcement that a person or group of affiliated or associated persons has become a 5% Shareholder (subject to certain exceptions described in the Plan) and (ii) 10 business days (or such later date as the Board shall determine) following the commencement of a tender offer or exchange offer that would result in a person or group of affiliated or associated persons becoming a 5% Shareholder (subject to certain exceptions described in the Plan).
The rights will expire at the earliest of (i) 5:00 P. M.  (New York City time) on September 5, 2017, or such later date and time (but not later than 5:00 P. M.  (New York City time) on September 5, 2019) as may be determined by the Board and approved by the stockholders of CF Holdings by a vote of the majority of the votes cast by the holders of shares entitled to vote thereon at a meeting of the stockholders of CF Holdings prior to 5:00 P. M.  (New York City time) on September 5, 2017, (ii) the time at which the rights are redeemed or exchanged as provided in the Plan, (iii) the time at which the Board determines that the Plan is no longer necessary or desirable for the preservation of Tax Benefits, and (iv) the close of business on the first day of a taxable year of CF Holdings to which the Board determines that no Tax Benefits may be carried forward.
In the event that a person or group of affiliated or associated persons becomes a 5% Shareholder (subject to certain exceptions described in the Plan), each holder of a right, other than such person, any member of such group or related person, all of whose rights will be null and void, will thereafter have the right to receive, upon exercise, common stock having a value equal to two times the exercise price of the right.
CF INDUSTRIES HOLDINGS, INC.  126 19.
Stock-Based Compensation 2014 Equity and Incentive Plan On May 14, 2014, our shareholders approved the CF Industries Holdings, Inc.  2014 Equity and Incentive Plan (the 2014 Equity and Incentive Plan) which replaced the CF Industries Holdings, Inc.  2009 Equity and Incentive Plan.
Under the 2014 Equity and Incentive Plan, we may grant incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock, restricted stock units, performance awards (payable in cash or stock) and other stock-based awards to our officers, employees, consultants and independent contractors (including non-employee directors).
The purpose of the 2014 Equity and Incentive Plan is to provide an incentive for our employees, officers, consultants and non-employee directors that is aligned with the interests of our stockholders.
Share Reserve and Individual Award Limits The maximum number of shares reserved for the grant of awards under the 2014 Equity and Incentive Plan is the sum of (i) 13.9 million and (ii) the number of shares subject to outstanding awards under our predecessor plans to the extent such awards terminate or expire without delivery of shares.
For purposes of determining the number of shares of stock available for grant under the 2014 Equity and Incentive Plan, each option or stock appreciation right is counted against the reserve as one share.
Each share of stock granted, other than an option or a stock appreciation right, is counted against the reserve as 1.61 shares.
If any outstanding award expires or is settled in cash, any unissued shares subject to the award are again available for grant under the 2014 Equity and Incentive Plan.
Shares tendered in payment of the exercise price of an option and shares withheld by the Company or otherwise received by the Company to satisfy tax withholding obligations are not available for future grant under the 2014 Equity and Incentive Plan.
As of December 31, 2016, we had 11.7 million shares available for future awards under the 2014 Equity and Incentive Plan.
The 2014 Equity and Incentive Plan provides that no more than 5.0 million underlying shares may be granted to a participant in any one calendar year.
Stock Options Under the 2014 Equity and Incentive Plan and our predecessor plans, we granted to plan participants nonqualified stock options to purchase shares of our common stock.
The exercise price of these options is equal to the market price of our common stock on the date of grant.
The contractual life of each option is ten years and generally one-third of the options vest on each of the first three anniversaries of the date of grant.
The fair value of each stock option award is estimated using the Black-Scholes option valuation model.
Key assumptions used and resulting grant date fair values are shown in the following table.
||2016|2015|2014|
|Weighted-average assumptions:||||
|Expected volatility|39%|31%|33%|
|Expected term of stock options|4.3 Years|4.3 Years|4.3 Years|
|Risk-free interest rate|1.2%|1.5%|1.3%|
|Expected dividend yield|3.3%|1.9%|1.6%|
|Weighted-average grant date fair value|$8.97|$13.99|$12.77|

The expected volatility of our stock options is based on the combination of the historical volatility of our common stock and implied volatilities of exchange traded options on our common stock.
The expected term of options is estimated based on our historical exercise experience, post-vesting employment termination behavior and the contractual term.
The risk-free interest rate is based on the U. S.  Treasury Strip yield curve in effect at the time of grant for the expected term of the options.
Ammonia Segment Our ammonia segment produces anhydrous ammonia (ammonia), which is our most concentrated nitrogen fertilizer as it contains 82% nitrogen.
The results of our ammonia segment consist of sales of ammonia to external customers.
In addition, ammonia is the "basic" nitrogen product that we upgrade into other nitrogen products such as granular urea, UAN and AN.
We produce ammonia at all of our nitrogen manufacturing complexes.
The following table presents summary operating data for our ammonia segment, including the impact of our acquisition of the remaining 50% equity interest in CF Fertilisers UK:
||Twelve months ended December 31,|
||2016|2015|2014|2016 v. 2015|2015 v. 2014|
||(in millions, except as noted)|
|Net sales|$981|$1,523|$1,576|$-542|-36%|$-53|-3%|
|Cost of sales|715|884|983|-169|-19%|-99|-10%|
|Gross margin|$266|$639|$593|$-373|-58%|$46|8%|
|Gross margin percentage|27.1%|42.0%|37.6%|-14.9%||4.4%||
|Sales volume by product tons (000s)|2,874|2,995|2,969|-121|-4%|26|1%|
|Sales volume by nutrient tons (000s)(1)|2,358|2,456|2,434|-98|-4%|22|1%|
|Average selling price per product ton|$341|$509|$531|$-168|-33%|$-22|-4%|
|Average selling price per nutrient ton-1|$416|$620|$648|$-204|-33%|$-28|-4%|
|Gross margin per product ton|$93|$213|$200|$-120|-56%|$13|7%|
|Gross margin per nutrient ton-1|$113|$260|$244|$-147|-57%|$16|7%|
|Depreciation and amortization|$96|$95|$69|$1|1%|$26|38%|
|Unrealized net mark-to-market loss (gain) on natural gas derivatives|$-85|$40|$25|$-125|N/M|$15|60%|

(1) Ammonia represents 82% nitrogen content.
Nutrient tons represent the tons of nitrogen within the product tons.
Year Ended December 31, 2016 Compared to Year Ended December 31, 2015 Net Sales.
Total net sales in the ammonia segment decreased by $542 million, or 36%, to $981 million in 2016 from $1.52 billion in 2015 due primarily to a 33% decrease in average selling prices and a 4% decrease in sales volume.
These results include the impact of the CF Fertilisers UK acquisition, which increased net sales by $26 million, or 2%.
The remaining decrease in our ammonia net sales of $568 million, or 37%, was due primarily to lower average selling prices and sales volume.
Selling prices declined due to excess global nitrogen supply.
In addition, our selling prices reflect the impact of a higher proportion of export sales, the volumes of which increased as a result of the weak fall application season attributable to the combined impact of weather conditions and low crop prices on our customers' decisions related to applying fertilizer in the fall.
Sales volume in 2016 declined due to combination of the weak fall application season and the impact of upgrading additional ammonia production at our Donaldsonville facility into granular urea and UAN as a result of our capacity expansion projects coming on line at our Donaldsonville, Louisiana complex.
Cost of Sales.
Cost of sales per ton in our ammonia segment averaged $248 per ton in 2016, including the impact of the CF Fertilisers UK acquisition, which averaged $220 per ton.
The remaining cost of sales per ton was $250 in 2016, a 16% decrease from the $296 per ton in 2015.
The decrease was due primarily to the impact of unrealized net mark-to-market gains on natural gas derivatives in 2016 compared to losses in 2015 and to the impact of lower realized natural gas costs in 2016.
This was partly offset by capacity expansion project start-up costs of $50 million and an increase in expansion project depreciation as a result of the new ammonia plants at our Donaldsonville and Port Neal facilities.

Question: what was the net change in airliner count during 2016?

Python solution:
def solution():
    # Define variables name and value
    new_aircraft = 55
    retired_aircraft = 71

    # Do math calculation to get the answer
    answer = new_aircraft - retired_aircraft

    return answer

Answer: -16