s = gets.chomp.split("")
s.sort!
s << s[-1]
m = 0
for i in 0..6
  if s[0] != s[1]
    puts s[0]
    break
  elsif s[2*i] != s[2*i-1]
    s[2*i] == s[2*i+1] ? m = s[2*i-1] : m = s[2*i]
  end
end
puts m