MIN X6-X1
SUBJECT TO
    X2 - X1 >= 1    
    X3 - X2 >= 2
    X4 - X2 >= 2
    X5 - X2 >= 4
    X5 - X4 >= 0
    X6 - X3 >=.5
    X6 - X5 >=.5
    X6 - X2 >= 1
END