The Fly

Two cyclists began a training run simultaneously, one starting from London, the other from Edinburgh.  When the riders were 180 m apart, a fly took an interest. Starting on one cyclist’s shoulder, the fly flew ahead to meet the other cyclist. On reaching the latter, fly at once turned back. The restless fly continued to shuttle back and forth until the pair met; then it settle on the nose of one of the cyclists.  The fly’s speed was 30 km per hour. Each cyclist’s speed was 15 km per hour. How far did the fly travel?