min 400X1+400X2+400X3+400X4+450Y1+450Y2+450Y3+450Y4+20I1+20I2+20I3+20I4
! Sailco example, Section 3.10
ST X1+Y1-I1=30
X2+Y2+I1-I2=60
X3+Y3+I2-I3=75
X4+Y4+I3-I4=25
X1<=40
X2<=40
X3<=40
X4<=40