set ORIG;
set DEST;
set ROADS = ORIG cross DEST;

param supply {ORIG} >= 0;   
param demand {DEST} >= 0;  
param cost {ROADS} >= 0;      

minimize Total_Cost;

node origins {k in ORIG}: net_out = supply[k];

node destinations {k in DEST}: net_in = demand[k];

arc X {(i,j) in ROADS} >= 0, from origins[i], to destinations[j], obj Total_Cost cost[i,j];