L = []
for _ in range(int(input())):
    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:
        c += C // 2
    else:
        c = -1
        break

print(c)