SUM column C when (column A='D' or column B='D'). Any of column cells contains 'D'.

In my excel sheet I have 3 columns as :

A        B        C
===================
XYZ      N      9
ABC      N      1
MNO    N      3
D                   13
D           D       7
D           D       9
D                    1

Now, From these sheet I want sum of all cells in C column where either value in A or B column's cell is 'D'.
something like : if(a='D' or b='D') then sum+=C

Here, for case SUM(where cell value is 'D' in columns A or B) will be 30