把一个字符串中所有出现的大写字母都替换成小写字母，同时把小写字母替换成大写字母。
输入一行：待互换的字符串。
输出一行：完成互换的字符串（字符串长度小于80）。
If so, you already have a Google Account. You can sign in on the right.
iF SO, YOU ALREADY HAVE A gOOGLE aCCOUNT. yOU CAN SIGN IN ON THE RIGHT.
D.字符串