4. Find a feasible solution or determine that no feasible solution exists for the following system of difference constraints: (15%)

$$x1 - x2 \le 1, x1 - x4 \le -4, x2 - x3 \le 2, x2 - x5 \le 7, x2 - x6 \le 5, x3 - x6 \le 10, x4 - x2 \le 2, x5 - x1 \le -1,$$

$$x5 - x4 \le 3, x6 - x3 \le 8.$$