program main
	implicit none
	integer i, j, counter
	character(1) cha(1:26)
	character(100000) S
	read(*, *) S
	cha(1)  = 'a'; cha(2)  = 'b'; cha(3)  = 'c'; cha(4)  = 'd'; cha(5)  = 'e'
	cha(6)  = 'f'; cha(7)  = 'g'; cha(8)  = 'h'; cha(9)  = 'i'; cha(10) = 'j'
	cha(11) = 'k'; cha(12) = 'l'; cha(13) = 'm'; cha(14) = 'n'; cha(15) = 'o'
	cha(16) = 'p'; cha(17) = 'q'; cha(18) = 'r'; cha(19) = 's'; cha(20) = 't'
	cha(21) = 'u'; cha(22) = 'v'; cha(23) = 'w'; cha(24) = 'x'; cha(25) = 'y'
	cha(26) = 'z'
	counter = 0
	do i = 1, 26
		do j = 1, len_trim(S)
			if(S(j:j) == cha(i)) then
				counter = counter + 1
				exit
			end if
		end do
		if(counter /= i) then
			write(*, *) cha(i)
			exit
		else if(counter == 26) then
			write(*, *) 'None'
			exit
		end if
	end do
end program main