Terms of trade
E can buy 1 kilo of cheese for five bottles of wine if they produce all by themselves
But W can buy 1 kilo of cheese for 2.33 bottles of wine (or five bottles buys 2.15 kilos of cheese [2.15=5/2.33])
Thus, E can gain an extra 1.15 by selling in W
W cheese producers can get more by selling in E