One day, a young man went to a shoe store and bought a pair of shoes. The cost of this pair of shoes was 15 yuan, and the price was 21 yuan. As a result, the young man took out 50 yuan to buy these shoes. The shoe store had no change at the time, so he exchanged the 50 yuan for 50 yuan change from the neighbor, and gave the young man 29 yuan. But the neighbor later found out that the 50 yuan was a counterfeit bill, and the shoe shop owner had no choice but to return the 50 yuan to the neighbor. How much did the shoe store lose on this transaction?
一天，有一个年轻人到鞋店里买了一双鞋，这双鞋的成本是15元，标价是21元．结果是这个年轻人掏出50元要买这双鞋，鞋店当时没有零钱，用那50元向街坊换了50元零钱，找给年轻人29元．但是街坊后来发现那50元是假钞，鞋店主无奈之下还给街坊50元．鞋店在这次交易中损失了多少元？
<gadget id="calculator">15 + 29</gadget>
<output>44</output>

<result>44</result>