def Entropy(text):
    import math
    log2=lambda x:math.log(x)/math.log(2)
    exr={}
    infoc=0
    for each in text:
        try:
            exr[each]+=1
        except:
            exr[each]=1
    textlen=len(text)
    for k,v in exr.items():
        freq  =  1.0*v/textlen
        infoc+=freq*log2(freq)
    infoc*=-1
    return infoc

while True:
    print Entropy(raw_input('>>>'))