MATLAB linear programming question

**Question>>>>**

Objective:

Max: 6 x1 + 10 x2 + 9 x3 + 20 x4

Constraints:

4x1 + 9x2 + 7x3 + 10x4 <= 600

x1 + x2 + 3x3 + 8x4 <= 420

30x1 + 40x2 + 20x3 + 10x4 <= 800

x1、 x2、x3、x4 >= 0

x1、 x2、x3、x4 = integer

**My Input>>>**

f = [-6, -10, -9, -20];

A = [4 9 7 10; 1 1 3 8; 30 40 20 10; -1 0 0 0; 0 -1 0 0; 0 0 -1 0; 0 0 0 -1];

b = [600; 420; 800; 0; 0; 0; 0];

simlp (f, A, b)

**then I got an answer:**

ans =

-0.0000

7.0968

0

51.6129

**but I want integer answer....where I did wrong??** (Crying)