diff --git a/extra/project-euler/186/186.factor b/extra/project-euler/186/186.factor
index 7504e09a81..b86f3675a1 100644
--- a/extra/project-euler/186/186.factor
+++ b/extra/project-euler/186/186.factor
@@ -1,7 +1,43 @@
-USING: circular disjoint-sets kernel math math.ranges
-sequences ;
+! Copyright (c) 2008 Eric Mertens.
+! See http://factorcode.org/license.txt for BSD license.
+USING: circular disjoint-sets kernel math math.ranges sequences ;
 IN: project-euler.186
 
+! http://projecteuler.net/index.php?section=problems&id=186
+
+! DESCRIPTION
+! -----------
+
+! Here are the records from a busy telephone system with one million users:
+
+!     RecNr  Caller  Called
+!     1      200007  100053
+!     2      600183  500439
+!     3      600863  701497
+!     ...    ...     ...
+
+! The telephone number of the caller and the called number in record n are
+! Caller(n) = S2n-1 and Called(n) = S2n where S1,2,3,... come from the "Lagged
+! Fibonacci Generator":
+
+! For 1 <= k <= 55, Sk = [100003 - 200003k + 300007k^3] (modulo 1000000)
+! For 56 <= k, Sk = [Sk-24 + Sk-55] (modulo 1000000)
+
+! If Caller(n) = Called(n) then the user is assumed to have misdialled and the
+! call fails; otherwise the call is successful.
+
+! From the start of the records, we say that any pair of users X and Y are
+! friends if X calls Y or vice-versa. Similarly, X is a friend of a friend of Z
+! if X is a friend of Y and Y is a friend of Z; and so on for longer chains.
+
+! The Prime Minister's phone number is 524287. After how many successful calls,
+! not counting misdials, will 99% of the users (including the PM) be a friend,
+! or a friend of a friend etc., of the Prime Minister?
+
+
+! SOLUTION
+! --------
+
 : (generator) ( k -- n )
     dup sq 300007 * 200003 - * 100003 + 1000000 rem ;
 
@@ -35,4 +71,7 @@ IN: project-euler.186
 : euler186 ( -- n )
     <generator> 0 1000000 <relation> (p186) ;
 
+! [ euler186 ] 10 ave-time
+! 18572 ms ave run time - 796.87 SD (10 trials)
+
 MAIN: euler186