#美しいWhiteSpace(1)

N = int(input())
s = []

for i in range(N):
    a = list(map(int , input().split()))
    s.append(a[0] + a[1] * 4)

s.sort()
max = s[-1]
c = 0

for i in s:
    if (max - i) % 2 == 0:
        c += (max - i) / 2
    else:
        c = -1
        break

print(int(c))