Evenly Separated Strings

문제

Given a string consisting of lowercase letters. We say the string is evenly separated if and only if there is an even number of characters between every pair of the same characters. For example, abba is evenly separated while aabba is not. For this problem, you will write a program to determine whether the given string s is evenly separated or not.

입력

Input consists of a single line containing the string of at most 1,000,000 lower-case letters to check if it is evenly separated.

출력

The output line consists of the word YES if the string is evenly separated or NO if the string is not evenly separated.