import strutils
let SA, SB = stdin.readLine.split

case SA[1].len - SB[1].len:
of 1 .. int.high: echo SA[0]
of int.low .. -1: echo SB[0]
else:
  for i, v in SA[1]:
    case ($SA[1][i]).parseInt - ($SB[1][i]).parseInt:
    of 1 .. int.high: echo SA[0]
    of int.low .. -1: echo SB[0]
    else: continue
    quit()
  echo -1