写的lingo的一个程序，求大神帮忙改错！

模型如图：

lingo程序如下，输入d的初始值为0或-10000000时，均出现报错，error code 81，无解。

想请教论坛上的大神是哪里有错误？

谢谢各位大神！

MODEL:

SETS:

DMU1/1,2/:S1,S2;  

DMU2/3,4/: S3,S4;

FACTOR/COST RICH WRIT SCIN/:W;

DXF1( DMU1, FACTOR): F1;

DXF2( DMU2, FACTOR): F2;

ENDSETS

DATA:

d=?;

F1= 1 2 3 4

2 3 4 5;

F2= 3 4 5 6

5 6 7 8;

ENDDATA

min =a+b;

@for(DMU1(J):

a=@SUM(DMU1(J):S1(J)));

@for(DMU2(J):

b=@SUM(DMU2(J):S4(J)));

@for(DMU1(J):

d+1=@SUM(DXF1(I,J):W(I)* F1(I,J))+S1(J)-S2(J));

@for(DMU2(J):

d=@SUM(DXF2(I,J):W(I)* F2(I,J))+S3(J)-S4(J));

@for(FACTOR(I):

1=@SUM(DXF1(I,J):W(I)));

@for(DMU1(J):

S1(J)>=0);

@for(DMU1(J):

S2(J)>=0);

@for(DMU2(J):

S3(J)>=0);

@for(DMU2(J):

S4(J)>=0);

END