diff --git a/phase1.dyalog b/phase1.dyalog
index 8cff9f3..00a66ea 100644
--- a/phase1.dyalog
+++ b/phase1.dyalog
@@ -14,3 +14,5 @@
  bits←{f←⍸∘⌽(2∘⊥⍣¯1)⋄∧/(f⍺)∊f⍵}
  ⍝ 8: Zigzag Numbers
  zigzag←∧/2=∘|2-/∘×2-/(10∘⊥⍣¯1)
+ ⍝ 9: Rise and Fall
+ risefall←{∧/(⍳∘≢≡⍋)¨(⊂((⊢⍳⌈/)↑⊢),⍵),⊂⌽((⊢⍳⌈/)↓⊢),⍵}