N = int(input().strip())

a = -1

for i in range(N):
  X, Y = (int(j) for j in input().strip().split(' '))
  
  if a < 0:
    a = Y - X
    if a < 0:
      print('-1')
      exit(1)
  else:
    if a != Y - X:
      print('-1')
      exit(1)
else:
  print(a)