N = int(raw_input())
raws = []
for i in range(0, N):
	a, b = map(int, raw_input().split())
	raws.append(a+4*b)
raws.sort(reverse=True)
sum = 0
for i in range(1, N):
	if raws[i]%2 != raws[0]%2:
		sum = -1
		break
	sum += (raws[0]-raws[i])/2
print sum