diff --git a/lib/fibonacci.rb b/lib/fibonacci.rb
index 606755b..86698c1 100644
--- a/lib/fibonacci.rb
+++ b/lib/fibonacci.rb
@@ -5,8 +5,23 @@
 # ....
 # e.g. 6th fibonacci number is 8
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n), n is the number of computations this calculation needs to run to compute the nth fibonacci number
+# Space complexity: O(1), space complexity is constant because this method only require a storage for 3 variables (p,p1,p2) regardless of number of operations
 def fibonacci(n)
-  raise NotImplementedError
+  if n.nil? || n < 0
+    raise ArgumentError
+  end
+  if n == 0
+    return 0
+  else
+    p1 = 0
+    p2 = 1
+
+    (n - 1).times do
+      p = p1 + p2
+      p1 = p2
+      p2 = p
+    end
+    return p2
+  end
 end