Excel or Google Sheets: Sum based on criteria in another column

I have a spreadsheet which contains two columns with two separate values for each row. Each letter indicates a value attached to a variable. The values of the variables change depending on which column they are in.

These are the values if the variable is in the column S1_ST: B=3, C=4, D=4, R=4, W=1, WA=0.

These are the values if the variable is in the column S2_ST: B=2, C=9, D=0, R=6, W=2, WA=0.

I am looking for a formula to return the sum of the value for S1_ST and S2_ST based on the variable entered in those columns.

For Example: https://drive.google.com/file/d/0Byxg512qAqFgX2RyQlh3ellJLTA/view?usp=sharing

So, the value in I2 should be 4.

Hope that makes sense.

Thanks in advance for the ideas.

Use vlookup.

You can use vlookup or sumifs. See my screenshot with the formula indicated. The number 3 at G1 is calculated from the list of values for the variables.