set EVAC;
set DEST;

set NODES := EVAC union DEST;
set EDGES within {NODES,NODES};

param population   {EVAC } >= 0;
param capacity     {DEST } >= 0;
param edgecapacity {EDGES} >= 0;

var Flow {(a,b) in EDGES} >= 0;

minimize TotalCost:
    + (sum {(a,b) in EDGES} Flow[a,b])
;

subject to EvacMaximumCapacity {n in EVAC}:
    + population[n] # population of city
    + sum {(a,n) in EDGES} Flow[a,n] # flow into city
    - sum {(n,b) in EDGES} Flow[n,b] # flow out of city
    <= 0
;
    
subject to DestMaximumCapacity {n in DEST}:
    + sum {(a,n) in EDGES} Flow[a,n] # flow into city
    - sum {(n,b) in EDGES} Flow[n,b] # flow out of city
    - capacity[n] # capacity of city to take population
    <= 0
;