N = int(input())
AB = [list(map(int, input().split())) for i in range(N)]

w = [a + 4 * b for a, b in AB]
odd = sum(x & 1 for x in w)
if odd in (0, N):
    ma = max(w)
    ans = sum(ma - x for x in w) // 2
else:
    ans = -1
print(ans)