鹬蚌相争，渔翁得利

yù bàng xiāng zhēng , yú wēng dé lìyubangxiangzheng,yuwengdeli

1. lit. when the snipe and the clam grapple with each other, the fisherman catches them both (idiom)
2. fig. when two sides are locked in dispute, it's often a third party that benefits

Study it in Yanamino