There are two children $\displaystyle C_1$ and $\displaystyle C_2$. $\displaystyle C_1$ has 12 different toys and $\displaystyle C_2$ has 13 different toys. Find the number of ways in which $\displaystyle C_1$ and $\displaystyle C_2$ can exchange their toys in such a way that after exchanging they still have same number of toys but not the same set.