Maximise the Tastiness

Chef is making a dish that consists of exactly two ingredients. He has four ingredients $A, B, C$ and $D$ with tastiness $a, b, c,$ and $d$ respectively. He can use either $A$ or $B$ as the first ingredient and either $C$ or $D$ as the second ingredient. The tastiness of a dish is the sum of tastiness of its ingredients. Find the **maximum** possible tastiness of the dish that the chef can prepa