180. Boring Strings
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output
This problem is worth 5 points.

You consider a string of text to be boring if it contains only "a" characters and "b" characters. For example, "abbaba" is boring, while "abcccbabc" is not boring.

Given a string, figure out whether or not it is a boring string.

Input

The only line of input contains a single string $$$s$$$, not containing spaces.

Output

If $$$s$$$ is boring, as described above, output "YES" (no quotes). Otherwise, output "NO".

Examples
Input
aaaaaa
Output
YES
Input
ccabbabbdd
Output
NO
Input
baaaaabad
Output
NO
Input
baaaaabba
Output
YES