Write the Following Expression as a Summation $$2 y_{3} f_{3}+2 y_{4} f_{4}+2 y_{5} f_{5}+2 y_{6} f_{6}$$

Write the following expression as a summation:
$$2 y_{3} f_{3}+2 y_{4} f_{4}+2 y_{5} f_{5}+2 y_{6} f_{6}$$