diff --git a/Algo/Courses/Chapter 7 - X (exercises).md b/Algo/Courses/Chapter 7 - X (exercises).md
index 3e56df0..c326c0c 100644
--- a/Algo/Courses/Chapter 7 - X (exercises).md	
+++ b/Algo/Courses/Chapter 7 - X (exercises).md	
@@ -1,8 +1,11 @@
 ```Ocaml
 # let sum n = 
-	if n = 0 then
-		0
-	else if n < 0 then
-		failwith "input must be positive"
+	if n < 0 then
+		invalid_arg "n<0"
 	else
-		n + sum(n-1)
\ No newline at end of file
+		let rec sumrc n  =
+			if n=0 then
+				0
+			else
+				n + sum(n-1)
+		in sumrc n;;
\ No newline at end of file