lingo解决钢管问题，错误24，求助

model:
sets:
cutfa/13/:x;
buj/14/:L,need;
shul(cutfa,buj):n;
endsets
data:
L=4 5 6 8;
need=50 10 20 15;
enddata
min=@sum(cutfa:x);
@for(buj(j):@sum(cutfa(i):n(i,j)x(i))>=need(j));
@for(cutfa(i):@sum(buj(j):n(i,j)L(j))<=19);
@for(cutfa(i):@sum(buj(j):n(i,j)L(j))>=16);
@for(shul:@gin(n));
@for(cutfa:@gin(x));
end