S = [(lambda a, b: int(a) + 4 * int(b))(*input().split()) for i in range(int(input()))]

m = max(S)
d = 0
for s in S:
    dd = m - s
    if dd % 2 != 0:
        print(-1)
        break
    else:
        d += dd
else:
    print(d // 2)