N = int(input().strip())
lst = [[int(i) for i in input().strip().split()] for _ in range(N)]
lst2 = [a + b * 4 for a, b in lst]
m = max(lst2)
if sum([(m - i) % 2 for i in lst2]):
  print(-1)
else:
  print(sum([(m - i) // 2 for i in lst2]))