diff --git a/strings/parenthesis.ipynb b/strings/parenthesis.ipynb
index 497f145..5c1214e 100644
--- a/strings/parenthesis.ipynb
+++ b/strings/parenthesis.ipynb
@@ -11,9 +11,18 @@
     "For example, given the string \"()())()\", you should return 1. Given the string \")(\", you should return 2, since we must remove all of them."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Approach\n",
+    "For each opening parenthesis to be considered valid, they should eventually be matched with an closing parenthesis. If not, they are counted as invalid.\n",
+    "Whenever we encounter an unmatched closing parenthesis, we count as invalid. Eventually, we'll add the unmatched open count to the invalid one."
+   ]
+  },
   {
    "cell_type": "code",
-   "execution_count": 26,
+   "execution_count": 32,
    "metadata": {},
    "outputs": [],
    "source": [
@@ -21,10 +30,10 @@
     "    invalid = 0\n",
     "    opened = 0\n",
     "    \n",
-    "    for parenthesis in string:\n",
-    "        if parenthesis == \"(\":\n",
+    "    for char in string:\n",
+    "        if char == \"(\":\n",
     "            opened += 1\n",
-    "        elif parenthesis == \")\":\n",
+    "        elif char == \")\":\n",
     "            if opened > 0:\n",
     "                opened -= 1\n",
     "            else:\n",
@@ -34,7 +43,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 28,
+   "execution_count": 33,
    "metadata": {},
    "outputs": [
     {
@@ -43,7 +52,7 @@
        "2"
       ]
      },
-     "execution_count": 28,
+     "execution_count": 33,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -52,6 +61,33 @@
     "count_invalid_parenthesis(\"((()(())\")"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": 34,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "0"
+      ]
+     },
+     "execution_count": 34,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "count_invalid_parenthesis(\"()()()\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This solution runs at O(N) time, where N = number of characters in the string."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,