fix comma positions

src/Addition.js

@@ -456,7 +456,11 @@ function Addition() {
 
     // if no commas in numbers
     if(commaPositions.length === 1 && commaPositions[0] !== numbersLen){
-      setCommaIdx(commaPositions[0] + 1);
+      if(commaPositions[0] < 0){
+         setCommaIdx(commaPositions[0]);
+      }else{
+        setCommaIdx(commaPositions[0] + 1);
+      }
       commaCorrect = true;
     }
 

src/Subtraction.js

@@ -509,7 +509,11 @@ function Subtraction() {
 
     // if no commas in numbers
     if(commaPositions.length === 1 && commaPositions[0] !== numbersLen){
-      setCommaIdx(commaPositions[0] + 1);
+      if(commaPositions[0] < 0){
+         setCommaIdx(commaPositions[0]);
+      }else{
+        setCommaIdx(commaPositions[0] + 1);
+      }
       commaCorrect = true;
     }