My result doesn't agree with results from online calculators

Question

I'm doing an exercise about linear programming, but my answer doesn't agree with the answer from online calculators like this and this. Below is the problem statement: $$Minimize: u = 4x - 3y, s.t.$$ $$y \le -x + 1$$ $$y \le x + 1$$ $$y \ge 0$$

I found 3 corner: (-1,0), (0,1), (1,0) and min = -4 at (-1,0), but these calculators gives me the answer -3 at (0,1). Did I miss anything when solving this problem?

Answer 1

The online calculators assume nonnegativity. If you write $x$ as the difference of two nonnegative numbers, your first calculator gives the right answer

Minimize p = 4x1-4x2-3y subject to
x1-x2+y <= 1
-x1+x2+y <= 1