func f(x, y: string): string =
  result = ""
  let
    lx = x.len
    ly = y.len
  for i in 0 ..< ly:
    result &= x[i]
    result &= y[i]
  if lx > ly:
    result &= x[^1]

let
  x, y = stdin.readLine
  l = x.len - y.len

echo:
  case l
  of 0, 1: f x, y
  else: "?"