def isValid(s):
    if len(s) <= 1:
        return "YES"
        
    freq = {}
    
    for i in s:
        freq[i] = freq.get(i, 0) + 1
    
    a = freq[s[0]]
    count_a = 0
    b = 0
    count_b = 0
    for i in freq:
        if freq[i] == a:
            count_a += 1
        elif not b:
            b = freq[i]
            count_b +=1
        elif freq[i] == b:
            count_b += 1
        else:
            return "NO"
    
    if count_a == 1:
        if a - b == 1 or a == 1:
            return "YES"
    elif count_b == 1:
        if b - a == 1 or b == 1:
            return "YES"
    elif count_a == 0 or count_b == 0:
        return "YES"
    
    return "NO"