 Complete the following phrase:  In 2015, the single mixed relay was introduced to the Biathlon World Cup by the IBU. The event is run on a track with a penalty loop, and each team consists of a female and a male runner. The race is divided into four legs, with the first three being or 2 laps and the final leg being or 3 laps, totalling . After each leg, the runners exchange so that each runner completes two legs. Specific to this format, the exchange happens immediately after the last shooting of each leg without skiing an additional lap as is usually the case. The race can be started by either the female or male member of the relay, with the finishing member performing an extra lap. This event was added to the world championships in 