diff --git a/Algo/Courses/Chapter 7 - X (exercises).md b/Algo/Courses/Chapter 7 - X (exercises).md
index ea549e1..3e56df0 100644
--- a/Algo/Courses/Chapter 7 - X (exercises).md	
+++ b/Algo/Courses/Chapter 7 - X (exercises).md	
@@ -1,2 +1,8 @@
 ```Ocaml
-# let double_succ
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+# let sum n = 
+	if n = 0 then
+		0
+	else if n < 0 then
+		failwith "input must be positive"
+	else
+		n + sum(n-1)
\ No newline at end of file