Sum Values Based On Text

I need a formula that will calculate the sum total of 6 and a longer formula to cope with 12 letters in a range. The third value in both ranges is worth double points.

The letters and their corresponding values are:

F=0, P=2, M=4, D=6

Some 6 letter examples to illustrate:

in A3:D3 the letters D,D,D,D,D,D return the value of 42 in E3 (6+6+12 (third letter is worth double)+6+6+6
in A4:D4 the letters P, P, F, F, M, M return the value of 12 in E4

Some 12 letter examples to illustrate:

in A3:L3 the letters D,D,D,D,D,D,D,D,D,D,D,D return the value of 78 (6+6+12+6+6+6+6+6+6+6+6+6)

If the range is empty the formula should leave the cell blank.