Assign different letters different values across a range of cells

Hi there, using Google Sheets here

I am inputting the letters G, A and R across D5 to N5

G is to assign the value 7
A is to assign the value 4
R is to assign the value 1

I dont need to see these values just the total please

I'd like the total of these values to total into Cell: D4

Please and thank you