Let $\alpha , \beta$ be the roots of quadratic equation $12 \mathrm { x } ^ { 2 } - 20 \mathrm { x } + 3 \lambda = 0$, $\lambda \in \mathbf { z }$. If $1 / 2 \leq | \beta - \alpha | \leq 3 / 2$ then the sum of all possible valued of $\lambda$ is $\_\_\_\_$ -