$${\Delta E_{se} = \Sigma(mc^2) - Mc^2 = 50(m_H c^2) + 70(m_n c^2) – M_{\mathrm{Sn}} c^2 \\ = 50(1,007825\, \mathrm{u})c^2 + 70(1,008665\, \mathrm{u})c^2 – (119,902197\, \mathrm{u})c^2 \\ = (1,095603\, \mathrm{u})c^2 = (1,095603\, \mathrm{u})(931,494013\, \mathrm{MeV/u}) \\ = 1020,5\, \mathrm{MeV}}$$