[R] Differenciate numbers from reference for rows

[M.Ribeiro]

So, I am having a tricky reference file to extract information from.

The format of the file is

x   1 + 4 * 3 + 5 + 6 + 11 * 0.5

So, the elements that are not being multiplied (1, 5 and 6) and the elements
before the multiplication sign (4 and 11) means actually the reference for
the row in a matrix where I need to extract the element from.

The numbers after the multiplication sign are regular numbers 
Ex:

> x<-matrix(20:35)
> x
      [,1]
 [1,]   20
 [2,]   21
 [3,]   22
 [4,]   23
 [5,]   24
 [6,]   25
 [7,]   26
 [8,]   27
 [9,]   28
[10,]   29
[11,]   30
[12,]   31
[13,]   32
[14,]   33
[15,]   34
[16,]   35

I would like to read the rows 1,4,5,6 and 11 and sum then. However the
numbers in the elements row 4 and 11 are multiplied by 3 and 0.5

So it would be
20 + 23 * 3 + 24 + 25 + 30 * 0.5.

And I have this format in different files so I can't do all by hand.
Can anybody help me with a script that can differentiate this?
Thanks
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