diff --git a/lib/fibonacci.rb b/lib/fibonacci.rb
index 606755b..a273e76 100644
--- a/lib/fibonacci.rb
+++ b/lib/fibonacci.rb
@@ -5,8 +5,25 @@
 # ....
 # e.g. 6th fibonacci number is 8
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n), where n is the the number of values in the array
+# Space complexity: O(c), where c is the integers that are being rewritten
+
 def fibonacci(n)
-  raise NotImplementedError
-end
+  if n == nil || n < 0
+    raise ArgumentError, "Invalid input for n, #{n}"
+  elsif n == 0 || n == 1
+    return n
+  else
+    fib_num_prev = 0 # 2
+    fib_num_curr = 1 # 3
+    fib_num_next = 0 # 3
+
+    (n-1).times do
+      fib_num_next = fib_num_prev + fib_num_curr
+      fib_num_prev = fib_num_curr
+      fib_num_curr = fib_num_next
+    end
+
+    return fib_num_next
+  end
+end
\ No newline at end of file