The total binding energy of an $\alpha \text{ - particle }\left({ }_2{\mathrm{He}}^4\right)\text{ is }24.4\mathrm{MeV}$ whereas the total binding energy of a deuteron $\left({ }_1{\mathrm{H}}^2\right)\text{ is merely }2.2\mathrm{MeV}$ When two deuterons are made to combine: 

${}_{1}H^2+{}_{1}H^2 \to {}_{2}H{e}^4 + Energy. In this case charge and mass numbers are conserved .$

Also energy released = Final total binding energy - Initial total binding energy =(24.4 - 2 $\times 2.2 ) Mev=20 0 Mev$