How to explain from the point of view of the law of conservation of energy that energy is released both during the fission

How to explain from the point of view of the law of conservation of energy that energy is released both during the fission of heavy nuclei and during the fusion of light nuclei?

Regardless of the type of reaction, energy is conserved. If the total rest energy of the particles entering into the reaction is greater than the total rest energy of the particles formed as a result of the reaction, then the energy is released. Energy cannot disappear.

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