diff --git a/lib/fibonacci.rb b/lib/fibonacci.rb
index 606755b..5e64efb 100644
--- a/lib/fibonacci.rb
+++ b/lib/fibonacci.rb
@@ -5,8 +5,29 @@
 # ....
 # e.g. 6th fibonacci number is 8
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: 0(n), where n is the given number
+# Space complexity: O(1)
 def fibonacci(n)
-  raise NotImplementedError
+  # raise NotImplementedError
+
+  if n == 0
+    return 0
+  end
+
+  if n == nil || n < 0
+    raise ArgumentError
+  else
+    a = 0
+    b = 1
+    for i in (1...n)
+      temp = b
+      b += a
+      a = temp
+    end
+    # Golden Ratio
+    # f = ((1.618034 ** n) - ((-0.618034) ** n)) / Math.sqrt(5)
+  end
+
+  return b
+  # return f.round(0)
 end