How To Sum Data Conditionally

I have a long row of data with words/numbers in each cell. Whenever I see a cell starting with W, I want to add up the value 2 cells to the right of that cell starting with W.

For example, if these following 9 cells in the same row read W1, 90, 100, M2, 90, 100, W2, 90, 40, whenever the cell starts with W, I want to sum up the value 2 cells to the right of that cell starting with W. In this case, I am adding up the values 2 cells to the right of W1 and W2, which is 100 + 40 = 140. How can I do that?