missing values

hello

i don’t know if it is possible to do it, but i have column with some missing values

i want to fill these values one by one, by taking in account the value added in the previous cell

to explain more

2010 5

2011 8

2012 10

2013 NA

2014 NA

2015 18

so i want to fill the 2013 value but when filling the 2014 it should take the 2013 value too