diff --git a/index.js b/index.js
index 713abd2..ecb9b0d 100644
--- a/index.js
+++ b/index.js
@@ -2,10 +2,11 @@ function add(a ,b){
     return a + b
 }
 function sub(a, b){
-    if(!(a>b)) throw new error (" no . a must be greater than b")
+    if(!(a>b)) throw new Error (" no . a must be greater than b")
     return a-b
 }
 function multi (a,b){
+    if (a<0 || b<0) throw new Error ("no a , b must be positive")
     return a*b
 }
 
@@ -18,3 +19,5 @@ mul(1 ,8)
 mul(4, 5)
 mul(6,8)
 mul(78,90)
+mul(8,9)
+mul(78,98)