trading or available-for-sale.
Both realized and unrealized gains and losses on equity securities classified as trading securities were recognized in net income.
For equity securities classified as available-for-sale, realized gains and losses were included in net income.
Unrealized gains and losses on equity securities classified as available-for-sale were recognized in accumulated other comprehensive income (loss) (“AOCI”), net of tax.
Equity securities without readily determinable fair values were recorded at cost.
Our investments in debt securities are carried at either amortized cost or fair value.
Investments in debt securities that the Company has the positive intent and ability to hold to maturity are carried at amortized cost and classified as held-to-maturity.
Investments in debt securities that are not classified as held-to-maturity are carried at fair value and classified as either trading or available-for-sale.
Refer to Note 4 for additional information on our policy for investments which include our assessment of impairments.
Trade Accounts Receivable We record trade accounts receivable at net realizable value.
This value includes an appropriate allowance for estimated uncollectible accounts to reflect any loss anticipated on the trade accounts receivable balances and charged to the provision for doubtful accounts.
We calculate this allowance based on our history of write-offs, the level of past-due accounts based on the contractual terms of the receivables, and our relationships with, and the economic status of, our bottling partners and customers.
We believe our exposure to concentrations of credit risk is limited due to the diverse geographic areas covered by our operations.
Activity in the allowance for doubtful accounts was as follows (in millions):
## Table 0 ##
1 The 2016 amount was primarily related to concentrate sales receivables from our bottling partner in Venezuela.
See “Hyperinflationary Economies” discussion below for additional information.2 Other includes foreign currency translation adjustments and the impact of reclassifying certain assets to assets held for sale.
Refer to Note 2.
A significant portion of our net operating revenues and corresponding accounts receivable is derived from sales of our products in international markets.
Refer to Note 3 and Note 20.
We also generate a significant portion of our net operating revenues by selling concentrates and syrups to bottlers in which we have a noncontrolling interest.
Refer to Note 7.
Derivative Instruments Our Company, when deemed appropriate, uses derivatives as a risk management tool to mitigate the potential impact of certain market risks.
The primary market risks managed by the Company through the use of derivative instruments are foreign currency exchange rate risk, commodity price risk and interest rate risk.
All derivatives are carried at fair value in our consolidated balance sheets in the following line items, as applicable: prepaid expenses and other assets; other assets; accounts payable and accrued expenses; and other liabilities.
The cash flow impact of the Company’s derivative instruments is primarily included in our consolidated statements of cash flows in net cash provided by operating activities.
Refer to Note 6.
Property, Plant and Equipment Property, plant and equipment are stated at cost.
Repair and maintenance costs that do not improve service potential or extend economic life are expensed as incurred.
Depreciation is recorded principally by the straight-line method over the estimated useful lives of our assets, which are reviewed periodically and generally have the following ranges: buildings and improvements: 40 years or less; and machinery, equipment and vehicle fleet: 20 years or less.
Land is not depreciated, and construction in progress is not depreciated until ready for service.
Leasehold improvements are amortized using the straight-line method over the shorter of the remaining lease term, including renewals that are deemed to be reasonably assured, or the estimated useful life of the improvement.
Depreciation is not recorded during the period in which a long-lived asset or disposal group is classified as held for sale, even if the asset or disposal group continues to generate revenue during the period.
Depreciation expense, including the depreciation expense of assets under capital lease, totaled $999 million, $1,131 million and $1,575 million in 2018, 2017 and 2016, respectively.
Amortization expense for leasehold improvements totaled $18 million, $19 million and $22 million in 2018, 2017 and 2016, respectively.
Refer to Note 8.
Average daily sales volumes and average realized sales prices were as follows:
## Table 1 ##
(1) Natural gas is converted on the basis of six Mcf of gas per one barrel of oil equivalent.
This ratio reflects an energy content equivalency and not a price or revenue equivalency.
Given recent commodity price disparities, the price for a barrel of oil equivalent for natural gas is less than the price for a barrel of oil.
(2) Average realized crude oil and condensate prices reflect reductions of $1.32 per Bbl for 2010, $2.13 per Bbl for 2009, and $22.06 per Bbl for 2008 from hedging activities.
Average realized natural gas prices reflect a decrease of $0.01 per Mcf for 2010 and an increase of $0.23 per Mcf for 2008 from hedging activities.
The effect of hedging activities on the average realized natural gas price for 2009 was de minimis.
These price increases and reductions resulted from hedge gains and losses that had been previously deferred AOCL.
All hedge gains or losses relating to US production had been reclassified to revenues by December 31, 2010.
(3) Average realized crude oil and condensate prices reflect reductions of $5.57 per Bbl for 2009 and $7.59 per Bbl for 2008 from hedging activities.
These price reductions resulted from hedge losses that had been previously deferred in AOCL.
All hedge gains or losses relating to Equatorial Guinea production had been reclassified to revenues by December 31, 2009.
(4) Natural gas from the Alba field in Equatorial Guinea is under contract for $0.25 per MMBtu to a methanol plant, an LPG plant and an LNG plant.
The methanol and LPG plants are owned by affiliated entities accounted for under the equity method of accounting.
(5) Includes production through November 24, 2010.
Our Block 3 PSC was terminated by the Ecuadorian government on November 25, 2010.
Intercompany natural gas sales were eliminated for accounting purposes.
Electricity sales are included in other revenues.
See Termination of Ecuador PSC, above.
(6) Volumes represent sales of condensate and LPG from the Alba Plant in Equatorial Guinea.
See Income from Equity Method Investees below.
Our debt issuances in 2014 were as follows:
## Table 2 ##
(a) Interest on these notes is payable annually in arrears beginning in March 2015.
(b) Interest on these notes is payable annually in arrears beginning in May 2015.
(c) Interest on these notes is payable annually in arrears beginning in December 2014.
(d) Interest on these notes is payable semiannually in arrears beginning in May 2015.
(e) U. S.  dollar equivalents for foreign currency notes were calculated based on exchange rates on the date of issuance.
The net proceeds from the sale of the securities listed in the table above will be used for general corporate purposes.
The weighted-average time to maturity of our long-term debt was 10.8 years at the end of 2013 and 2014.  ?
Off-Balance Sheet Arrangements and Aggregate Contractual Obligations We have no off-balance sheet arrangements, including special purpose entities, other than guarantees and contractual obligations discussed below.
Guarantees – At December 31, 2014, we were contingently liable for $1.0 billion of guarantees of our own performance, which were primarily related to excise taxes on the shipment of our products.
There is no liability in the consolidated financial statements associated with these guarantees.
At December 31, 2014, our third-party guarantees were insignificant.

Tables referenced above, in order (Table 0 first):
<table><tr><td>Year Ended December 31,</td><td>2018</td><td>2017</td><td>2016</td></tr><tr><td>Balance at beginning of year</td><td>$477</td><td>$466</td><td>$352</td></tr><tr><td>Net charges to costs and expenses<sup>1</sup></td><td>29</td><td>32</td><td>126</td></tr><tr><td>Write-offs</td><td>-4</td><td>-10</td><td>-10</td></tr><tr><td>Other<sup>2</sup></td><td>-13</td><td>-11</td><td>-2</td></tr><tr><td>Balance at end of year</td><td>$489</td><td>$477</td><td>$466</td></tr></table>
<table><tr><td></td><td colspan="4">Sales Volumes</td><td colspan="3">Average Realized Sales Prices</td></tr><tr><td></td><td>Crude Oil &Condensate(MBbl/d)</td><td>Natural Gas(MMcf/d)</td><td>NGLs(MBbl/d)</td><td>Total(MBoe/d)(1)</td><td>Crude Oil &Condensate(Per Bbl)</td><td>Natural Gas(Per Mcf)</td><td>NGLs(Per Bbl)</td></tr><tr><td>Year Ended December 31, 2010</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>United States-2</td><td>39</td><td>400</td><td>14</td><td>119</td><td>$75.03</td><td>$4.17</td><td>$41.21</td></tr><tr><td>Equatorial Guinea-3 (4)</td><td>11</td><td>226</td><td>-</td><td>49</td><td>78.44</td><td>0.27</td><td>-</td></tr><tr><td>Israel</td><td>-</td><td>130</td><td>-</td><td>22</td><td>-</td><td>4.03</td><td>-</td></tr><tr><td>North Sea</td><td>10</td><td>6</td><td>-</td><td>11</td><td>80.24</td><td>5.35</td><td>-</td></tr><tr><td>Ecuador-5</td><td>-</td><td>25</td><td>-</td><td>4</td><td>-</td><td>-</td><td>-</td></tr><tr><td>China</td><td>4</td><td>-</td><td>-</td><td>4</td><td>75.15</td><td>-</td><td>-</td></tr><tr><td>Total Consolidated Operations</td><td>64</td><td>787</td><td>14</td><td>209</td><td>76.46</td><td>3.00</td><td>41.21</td></tr><tr><td>Equity Investees-6</td><td>2</td><td>-</td><td>5</td><td>7</td><td>77.98</td><td>-</td><td>53.68</td></tr><tr><td>Total</td><td>66</td><td>787</td><td>19</td><td>216</td><td>$76.50</td><td>$3.00</td><td>$44.90</td></tr><tr><td>Year Ended December 31, 2009</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>United States-2</td><td>37</td><td>397</td><td>10</td><td>113</td><td>$55.19</td><td>$3.61</td><td>$27.96</td></tr><tr><td>Equatorial Guinea-3 (4)</td><td>14</td><td>239</td><td>-</td><td>54</td><td>55.94</td><td>0.27</td><td>-</td></tr><tr><td>Israel</td><td>-</td><td>114</td><td>-</td><td>19</td><td>-</td><td>3.47</td><td>-</td></tr><tr><td>North Sea</td><td>7</td><td>5</td><td>-</td><td>8</td><td>59.51</td><td>5.75</td><td>-</td></tr><tr><td>Ecuador</td><td>-</td><td>26</td><td>-</td><td>4</td><td>-</td><td>-</td><td>-</td></tr><tr><td>China</td><td>4</td><td>-</td><td>-</td><td>4</td><td>54.40</td><td>-</td><td>-</td></tr><tr><td>Total Consolidated Operations</td><td>62</td><td>781</td><td>10</td><td>202</td><td>55.76</td><td>2.54</td><td>27.96</td></tr><tr><td>Equity Investees-6</td><td>2</td><td>-</td><td>6</td><td>8</td><td>59.51</td><td>-</td><td>36.03</td></tr><tr><td>Total</td><td>64</td><td>781</td><td>16</td><td>210</td><td>$55.87</td><td>$2.54</td><td>$31.20</td></tr><tr><td>Year Ended December 31, 2008</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>United States-2</td><td>40</td><td>395</td><td>9</td><td>116</td><td>$75.53</td><td>$8.12</td><td>$50.15</td></tr><tr><td>Equatorial Guinea-3 (4)</td><td>15</td><td>206</td><td>-</td><td>49</td><td>88.95</td><td>0.27</td><td>-</td></tr><tr><td>Israel</td><td>-</td><td>139</td><td>-</td><td>23</td><td>-</td><td>3.10</td><td>-</td></tr><tr><td>North Sea</td><td>10</td><td>5</td><td>-</td><td>11</td><td>100.56</td><td>10.54</td><td>-</td></tr><tr><td>Ecuador</td><td>-</td><td>22</td><td>-</td><td>4</td><td>-</td><td>-</td><td>-</td></tr><tr><td>China</td><td>4</td><td>-</td><td>-</td><td>4</td><td>82.66</td><td>-</td><td>-</td></tr><tr><td>Total Consolidated Operations</td><td>69</td><td>767</td><td>9</td><td>207</td><td>82.60</td><td>5.04</td><td>50.15</td></tr><tr><td>Equity Investees-6</td><td>2</td><td>-</td><td>6</td><td>8</td><td>96.77</td><td>-</td><td>58.81</td></tr><tr><td>Total</td><td>71</td><td>767</td><td>15</td><td>215</td><td>$82.96</td><td>$5.04</td><td>$53.45</td></tr></table>
<table><tr><td>Type</td><td></td><td>Face Value<sup>(e)</sup></td><td>Interest Rate</td><td>Issuance</td><td>Maturity</td></tr><tr><td>EURO notes</td><td><sup>(a)</sup></td><td>€750 (approximately $1,029)</td><td>1.875%</td><td>March 2014</td><td>March 2021</td></tr><tr><td>EURO notes</td><td><sup>(a)</sup></td><td>€1,000 (approximately $1,372)</td><td>2.875%</td><td>March 2014</td><td>March 2026</td></tr><tr><td>EURO notes</td><td><sup>(b)</sup></td><td>€500 (approximately $697)</td><td>2.875%</td><td>May 2014</td><td>May 2029</td></tr><tr><td>Swiss franc notes</td><td><sup>(c)</sup></td><td>CHF275 (approximately $311)</td><td>0.750%</td><td>May 2014</td><td>December 2019</td></tr><tr><td>Swiss franc notes</td><td><sup>(b)</sup></td><td>CHF250 (approximately $283)</td><td>1.625%</td><td>May 2014</td><td>May 2024</td></tr><tr><td>U.S. dollar notes</td><td><sup>(d)</sup></td><td>$500</td><td>1.250%</td><td>November 2014</td><td>November 2017</td></tr><tr><td>U.S. dollar notes</td><td><sup>(d)</sup></td><td>$750</td><td>3.250%</td><td>November 2014</td><td>November 2024</td></tr><tr><td>U.S. dollar notes</td><td><sup>(d)</sup></td><td>$750</td><td>4.250%</td><td>November 2014</td><td>November 2044</td></tr></table>

Question: What do all NGLs(MBbl/d) sum up, excluding those negative ones in 2009?
Answer: 16
Reasoning program: add(const_10,const_6)