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Use these hints if you're stuck. Try solving on your own first.
For each node <code>u</code> in the first tree, find the number of nodes at a distance of at most <code>k</code> from node <code>u</code>.
For each node <code>v</code> in the second tree, find the number of nodes at a distance of at most <code>k - 1</code> from node <code>v</code>.