func main()
	var ss: [][]char
	do ss :: cui@input().split(" ")
	var sa: []char :: ss[0]
	var pa: []char :: ss[1]
	do ss :: cui@input().split(" ")
	var sb: []char :: ss[0]
	var pb: []char :: ss[1]
	
	var ans: []char :: "-1"
	var res: int :: f(sa, sb)
	if(res > 0)
		do ans :: sa
	elif(res < 0)
		do ans :: sb
	end if
	do cui@print("\{ans}\n")
	
	func f(sa: []char, sb: []char): int
		if(^sa <> ^sb)
			ret ^ sa - ^sb
		end if
		for i(0, ^sa - 1)
			if(sa[i] $ int <> sb[i] $ int)
				ret sa[i] $ int - sb[i] $ int
			end if
		end for
	end func
end func