Two trains, one travelling at $15{\text{\hspace{0.17em}ms}}^{-1}$ and other at  $20{\text{\hspace{0.17em}ms}}^{-1},$ are heading towards one another along a straight track. Both the drivers apply brakes simultaneously when they are 500 m apart. If each train has a retardation of $1{\text{\hspace{0.17em}ms}}^{-2},$  find the separation after they stop. Give answer in meter to nearest integer.