N = int(input())
all_sum = 0
list = []
for i in range(N):
    a, b = map(int, input().split())
    sum_b = b * 4
    sum = a + sum_b
    list.append(sum)
    all_sum += sum
num = (len(list) - 1) * max(list) - (all_sum - max(list))
if num % 2 == 1:
    print("-1")
else:
    ans = num / 2
    print(int(ans))