21. Five kittens are having a fishing competition, the more fish they catch, the higher their ranking; if they catch the same number of fish, their ranking is tied. There are a total of 100 fish in the pond. Now, the five kittens have caught 23, 20, 15, 18, and 13 fish respectively, and the kitten named Xiaohua is in second place. To ensure that Xiaohua ultimately ranks first, it needs to catch at least $\qquad$ more fish.
\begin{tabular}{c}
21 \\
\hline 7
\end{tabular}