  We report the first calculation using physical light-quark masses of the
electromagnetic form factor $V(z)$ describing the long-distance contributions
to the $K^+\to\pi^+\ell^+\ell^-$ decay amplitude. The calculation is performed
on a 2+1 flavor domain wall fermion ensemble with inverse lattice spacing
$a^{-1}=1.730(4)$GeV. We implement a Glashow-Iliopoulos-Maiani cancellation by
extrapolating to the physical charm-quark mass from three below-charm masses.
We obtain $V(z=0.013(2))=-0.87(4.44)$, achieving a bound for the value. The
large statistical error arises from stochastically estimated quark loops.
