import sequtils, strutils

let
  n = parseInt readLine stdin
  xys = n.newSeqWith stdin.readLine.split.map parseInt
  ans = xys[0][1].pred xys[0][0]

if ans < 1:
  echo -1
  quit()

for xy in xys:
  let
    x = xy[0]
    y = xy[1]
    
  if y.pred(x) != ans:
    echo -1
    quit()

echo ans