import strutils, sequtils
var AB = stdin.readLine.split

for i, v in AB:
  AB[i] = align(v, AB.mapIt(it.len).max, '0')

for i in 0 ..< AB[0].len:
  var j = 0
  if AB[0][i] == '4' or AB[0][i] == '7':
    if AB[1][i] == '4' or AB[1][i] == '7':
      j = 1

  case AB[0][i].int - AB[1][i].int
  of 0:
    continue
  of 1 .. int.high:
    echo AB[j].parseInt
  else:
    echo AB[j xor 1].parseInt
  quit()