Two identical air$-$filled parallel$-$plate capacitors C$1$C$1$ and C$2$C$2,$ each with capacitance CC$,$ are connected in series to a battery that has voltage VV$.$ While the two capacitors remain connected to the battery, a dielectric with dielectric constant K$>1$K$>1$ is inserted between the plates of one of the capacitors, completely filling the space between them. Let U$0$U$0$ be the total energy stored in the two capacitors without the dielectric and UU be the total energy stored after the dielectric is inserted. What is the ratio U$/$U$0$U$/$U$0?$