def isValid(s):
    Dict = createFirstDict(s)
    Dict2 = createSecondDict(Dict)
    res = determineIfIsValid(Dict2)
    return res
    
def createFirstDict(s):
    res = {}
    for ch in s:
        if ch not in res:
            res[ch] = 1
        else:
            res[ch] +=1
            
    return res

def createSecondDict(Dict):
    res = {}
    for key1 in Dict:
         if Dict[key1] not in res:
            res[Dict[key1]] = 1
         else:
            res[Dict[key1]] += 1
            
    return res
    
def determineIfIsValid(Dict2):
    if len(Dict2) == 1:
        return "YES"
    elif len(Dict2) > 2:
        return "NO"
    else:
        keyList = list(Dict2.keys())
        key1 = keyList[0]
        key2 = keyList[1]
        value1 = Dict2[key1]
        value2 = Dict2[key2]
        maxKey = max(key1, key2)
        minKey = min(key1, key2)
        if maxKey - minKey == 1 and Dict2[maxKey] == 1:
            return "YES"
        elif key1 == value1 == 1 or key2 == value2 == 1:
				
            return "YES"
        
    return "NO"