def CHECK(N, XYS):
  if XYS[0][0] >= XYS[0][1]:
    return False
  S = XYS[0][1] - XYS[0][0]
  for I in range(1, N):
    if XYS[I][0] > XYS[I][1]:
      return False
    else:
      if S != (XYS[I][1] - XYS[I][0]):
        return False
  return True
N = int(input())
XYS = []
for _ in range(N):
  Xi, Yi = map(int, input().split())
  XYS.append([Xi, Yi])
if CHECK(N, XYS):
  print(XYS[0][1] - XYS[0][0])
else:
  print(-1)