Assume that there are $N$ different bins and two different kinds of balls, $R$ red balls and $W$ white balls. 

The red balls and the white balls are randomly distributed across the bins (that is, for each ball it is equally likely to end up in each bin). Denote by $R_i$ and $W_i$ the numbers of red and white balls in bin $i$, $i=1,...,N$.

What is the probability distribution of $X=\sum_{i=1}^{N}{min(R_i,W_i)}$?