  Assuming that the differential rotation of the massive neutron star (NS)
formed in the double NS (DNS) mergers has been effectively terminated by the
magnetic braking and a uniform rotation has been subsequently established
(i.e., a supramassive NS is formed), we analytically derive in this work an
approximated expression for the critical total gravitational mass ($M_{\rm
tot,c}$) to form supramassive NS (SMNS) in the DNS mergers, benefited from some
equation of state (EoS) insensitive relationships. The maximum gravitational
mass of the nonrotating NSs ($M_{\rm TOV}$) as well as the dimensionless
angular momentum of the remnant ($j$) play the dominant roles in modifying
$M_{\rm tot,c}$, while the radius and mass differences of the premerger NSs do
not. The GW170817/GRB 170817A/AT2017gfo observations have provided so far the
best opportunity to quantitatively evaluate $M_{\rm TOV}$. Supposing the
central engine for GRB 170817A is a black hole quickly formed in the collapse
of an SMNS, we find $M_{\rm TOV}=2.13^{+0.09}_{-0.08}M_\odot$ (68.3%
credibility interval, including also the uncertainties of the EoS insensitive
relationships), which is consistent with the constraints set by current NS mass
measurements.
