The derivative of $$f(x, y)$$ at point $$(1, 2)$$ in the direction of vector $$\overrightarrow i + \overrightarrow j $$ is $$2\sqrt 2 $$ and in the direction of the vector $$ - 2\overrightarrow j $$ is $$-3.$$ Then the derivative of $$f(x,y)$$ in direction $$ - \overrightarrow i - 2\overrightarrow j $$ is