If condition to count column values based on index number

I have 14 column with 1st column (say A) containing the month value (1 for Jan, 2 for Feb and so on). Colum B contain a number representing 2017 year value. For the next 12 columns starting column C onwards, there are values for each month of 2018 starting from Jan 2018 till Dec 2018. In Col 15, i want to sum value of 2017 (Column B) with corresponding months in 2018 based on the month value in Column A. For example, if the month value is 3, then the result should give me the sum of values in column B(2017) + column C(Jan2018) + column D(Feb 2018) + column B (Mar 2018). Since month value 3 represents March 2018. I tried to use the function Choose but seems too length. Is there a smaller and easy function?