25. Put $2 n$ distinct items into $m$ distinct boxes, so that the number of items in each box is even (zero is also considered even), find the number of different ways to do this $f_{2 n}(m)$.
25. $f_{2 n}(m)=\frac{1}{2^{m}} \sum_{k=0}^{m}\binom{m}{k}(2 k-m)^{2 n}$.