The volume of the tetrahedron having the edges $\overrightarrow {i}+2 \overrightarrow {j}-\overrightarrow {k},\overrightarrow {i}+\overrightarrow {j}+ \overrightarrow {k},\overrightarrow {i}+\overrightarrow {j}+ \lambda \overrightarrow {k}$ as coterminous, is $\large\frac{2}{3} $ cubic units. Then $\lambda=$