If x and y be two non-zero vectors such that | x + y |=| x | and 2 x +λ y is perpendicular to y, then the value of λ is \ \ \ \.

If $\displaystyle \vec{x}$ and $\displaystyle \vec{y}$ be two non-zero vectors such that $\displaystyle |\vec{x}+\vec{y}|=|\vec{x}|$ and $\displaystyle 2 \vec{x}+\lambda \vec{y} \quad$ is perpendicular to $\displaystyle \vec{y}$, then the value of $\displaystyle \lambda$ is \_\_\_\_.