Please check test #20 for this problem. It seems to be incorrect. There is a pulley X which is connected to Y, but Y is not connected to X. I think so, because i got WA20, then added such code and got AC.
Code:
for (i = 0; i < n; i++)
    {
        if (blk[i].y > 0)
        {
            int t = blk[i].y - 1;
            if ((blk[t].x1 != i + 1) && (blk[t].y != i + 1) && (blk[t].x2 != i + 1))
            {
                cout << "1.0000";
                return;
            }
        }
        if (blk[i].x1 > 0)
        {
            int t = blk[i].x1 - 1;
            if ((blk[t].x1 != i + 1) && (blk[t].y != i + 1) && (blk[t].x2 != i + 1))
            {
                cout << "1.0000";
                return;
            }
        }
        if (blk[i].x2 > 0)
        {
            int t = blk[i].x2 - 1;
            if ((blk[t].x1 != i + 1) && (blk[t].y != i + 1) && (blk[t].x2 != i + 1))
            {
                cout << "1.0000";
                return;
            }
        }
    }