N = int(raw_input())
c = [0 for i in range(N)]
mi = 9999
ma = 0
for i in range(N):
	a,b = map(int,raw_input().split())
	c[i] = a + b * 4
	ma = max(ma,c[i])
	mi = min(mi,c[i])
ans = 0
for i in range(N):
	if (ma - c[i]) % 2 == 0:
		ans += (ma - c[i]) / 2
	else:
		ans = -1
		break
print ans