Being a secret computer geek, Farmer John labels all of his cows with
binary numbers. However, he is a bit superstitious, and only labels cows
with binary numbers that have exactly K "1" bits (1 <= K <= 10). The
leading bit of each label is always a "1" bit, of course. FJ assigns
labels in increasing numeric order, starting from the smallest possible
valid label -- a K-bit number consisting of all "1" bits. Unfortunately,
he loses track of his labeling and needs your help: please determine the Nth label he should assign (1 <= N <= 10^7).
作为一个神秘的电脑极客，农夫约翰使用二进制数字给他的所有奶牛编号。
但是，他有点迷信，他只使用刚好包含 K 个 1 的二进制数给奶牛编号。
当然，每个编号的前导位始终是 1。
约翰从最小的有效编号（K 个 1 构成的二进制数）开始，以递增的数字顺序给每头奶牛分配编号。
请确定他应该分配的第 N 个编号。