#shortest part
param numNodes;
set Nodes := 1..numNodes;

param cost{i in Nodes,j in Nodes};
param demand{i in Nodes};

var flow{i in Nodes,j in Nodes} >= 0;
minimize tripcost: sum{i in Nodes,j in Nodes} flow[i,j] * cost[i,j];

s.t. conservation{i in Nodes}:sum{h in Nodes} flow[h,i]-sum{j in Nodes} flow[i,j] >= demand[i];