#python 3
def isValid(s):
    keys = set(s)
    frequencies = {key:0 for key in keys}
    for letter in s:
        frequencies[letter] += 1
        
    counts = Counter(frequencies.values())
    maxCount = max(counts.values())
    mode = min([key for key, value in counts.items() if value == maxCount])
    diff = 0
    
    for letter,num in frequencies.items():
        if num != mode:
            if (abs(num - mode) == 1 or num == 1) and diff == 0:
                diff += 1
            else:
                return "NO"
    return "YES"