Heisenberg Hamiltonian for 2 Electron System: Get Relation (1)

[LagrangeEuler]

For two electron system $\vec{S}_1$, $\vec{S}_2$
$$\mathcal{H}=J\vec{S}_1\cdot \vec{S}_2=J(\frac{1}{2}(S_{tot})^2-\frac{3}{4}) (1)$$
How you get relation (1)?

 

[M Quack]

hint: $(\vec{S}_1+\vec{S}_2)^2$