#!/bin/python3

import os
from collections import Counter
from typing import Literal

#
# Complete the 'is_valid' function below.
#
# The function is expected to return a STRING.
# The function accepts STRING s as parameter.
#


def is_valid(s: str) -> Literal["NO", "YES"]:
    """Given a string, determine if it is valid."""
    y = "YES"
    if len(s) == 1:  # optional
        return y
    match len(counts := {*(frequencies := Counter(s).values())}):
        case 1:
            return y
        case 2:
            meta_freq = Counter(frequencies)
            lo, hi = sorted(counts)
            if 1 == lo == meta_freq[lo]:
                return y
            if 1 == hi - lo == meta_freq[hi]:
                return y
    return "NO"


if __name__ == "__main__":
    result = is_valid(input())
    with open(os.environ["OUTPUT_PATH"], "w") as fptr:
        fptr.write(f"{result}\n")