N = int(input())

L = []
for _ in range(N):
    a, b = map(int, input().split())
    L += [a + 4*b]

M = max(L)
c = 0
for i in L:
    C = M - i
    if C % 2 != 0:
        print(-1)
        exit()
    else:
        c += C

print(c//2)