 Complete the following phrase:  Another use of the duathlon is in cross country skiing where skiers have a mass start using the classical technique and ski a specific distance men - 15 km, women - 7.5 km, then change their skis, poles, and boots in a pit stop manner similar to a triathlon, then ski at the same specific distance men - 15 km, women - 7.5 km in the freestyle skating technique. The winner of the event is the competitor who crosses the finish line first. It was first introduced at the FIS Nordic World Ski Championships 2003 in Val di Fiemme as a 10 km  10 km event for men, 5 km  5 km event for women. The distances for the double pursuit were lengthened to their current length at the following 