It is known that A, B, C, D and A+C, B+C, B+D, D+A respectively represent the eight natural numbers 1 to 8, and they are not equal to each other. If A is the largest number among the four numbers A, B, C, and D, what is A?
已知A，B，C，D和A+C，B+C，B+D，D+A分别表示1至8这八个自然数，且互不相等．如果A是A，B，C，D这四个数中最大的一个数，那么A是多少．
<gadget id="calculator">1 + 2 + 3</gadget>
<output>6</output>

<result>6</result>