s = raw_input()
lmax = 0
ltemp = 0
sout = ''
stemp = ''
for i in xrange(len(s)):
    if s[i] in stemp:
        p = stemp.find(s[i])
        stemp = stemp[p + 1:]
        stemp += s[i]
        ltemp = len(stemp)
    else:
        stemp += s[i]
        ltemp += 1
    if lmax < ltemp:
        lmax = ltemp
        sout = stemp
print sout