diff --git a/package.json b/package.json
index 1263eee..af25104 100644
--- a/package.json
+++ b/package.json
@@ -63,4 +63,4 @@
     "express": "^4.18.1",
     "zod": "^3.17.10"
   }
-}
+}
\ No newline at end of file
diff --git a/src/01-number.md b/src/01-number.md
new file mode 100644
index 0000000..d336119
--- /dev/null
+++ b/src/01-number.md
@@ -0,0 +1,30 @@
+# Parameter X implicitly has an 'any' type
+
+```typescript
+const addTwoNumbers = (a, b) => {
+   return a + b
+}
+```
+
+> Parameter 'a' implicitly has an 'any' type.
+>
+> Parameter 'b' implicitly has an 'any' type.
+
+## What is going on?
+
+Missing type annotation to a parameter in a function.
+
+It is turned on by strict mode.
+
+```json
+// tsconfig.json
+{
+  "compilerOptions": {
+    "strict": true
+  }
+}
+```
+
+## Why is this important?
+
+This is the essence of Typescript, without it the code is loosely typed and errors are prone to happen.
diff --git a/src/01-number.problem.ts b/src/01-number.problem.ts
index 0f6286e..f206234 100644
--- a/src/01-number.problem.ts
+++ b/src/01-number.problem.ts
@@ -1,6 +1,6 @@
 import { expect, it } from "vitest";
 
-export const addTwoNumbers = (a, b) => {
+export const addTwoNumbers = (a: number, b:number) => {
   return a + b;
 };