def f(x):
    return int(x[0]) + int(x[1]) * 4
N = int(input())
s = [f(input().split()) for i in range(N)]
m = max(s)
a = 0
for i in s:
    d = max(0, m - i)
    if d % 2:
        print(-1)
        exit(0)
    a += d // 2
print(a)