n = int(input())
xy = {}
inds = set()

di = [1, -1, 2, 2, 1, -1, -2, -2]
dj = [2, 2, 1, -1, -2, -2, 1, -1]
for i in range(n):
    x, y = map(int, input().split())
    xy[(x, y)] = i
    for k in range(8):
        inds.add((x + di[k], y + dj[k]))


dp = [1 << 30] * (1 << n)
dp[0] = 0


for x, y in inds:
    if (x, y) in xy:
        continue
    ok = []
    for k in range(0, 8, 2):
        x1 = x + di[k]
        y1 = y + dj[k]
        x2 = x + di[k + 1]
        y2 = y + dj[k + 1]
        if (x1, y1) in xy and (x2, y2) in xy:
            ok.append((1 << xy[(x1, y1)]) | (1 << xy[(x2, y2)]))

        if (x1, y1) in xy:
            ok.append(1 << xy[(x1, y1)])
        if (x2, y2) in xy:
            ok.append(1 << xy[(x2, y2)])

    for bit in range((1 << n) - 1, -1, -1):
        for bit2 in ok:
            if (bit & bit2) == bit2:
                dp[bit] = min(dp[bit], dp[bit ^ bit2] + 1)

if dp[-1] > n:
    print(-1)
else:
    print(dp[-1])