func main()
	var x: []char :: cui@input()
	var y: []char :: cui@input()
	
	var ans: []char :: ""
	switch(^x - ^y)
	case 0, 1
		var i: int :: 0
		var j: int :: 0
		while loop(true)
			if(i > ^x - 1)
				break loop
			end if
			do ans :~ [x[i]]
			do i :+ 1
			
			if(j > ^y - 1)
				break loop
			end if
			do ans :~ [y[j]]
			do j :+ 1
		end while
	default
		do ans :: "?"
	end switch
	do cui@print("\{ans}\n")
end func