n = int(input())
l = []
sum_list = []
space_sum = 0
for i in range(n):
    l.append(list(map(int, input().split())))

for i in range(len(l)):
    sum_list.append(l[i][0] + l[i][1] * 4)

max = max(sum_list)
def judge(array):
    for i in sum_list:
        global space_sum
        if (max - i) % 2 == 0:
            space_sum += (max - i) / 2
        else:
            return False
    return True

if judge(sum_list):
    print(int(space_sum))
else:
    print('-1')