I'm having hard time regarding how to model a constraint. I'm trying to solve a transportation problem and one of the constraints is as follows:

t[s[i]] >= t[i] + travelTime (i, s[i])

which t is the time at which node i is visited and s is the successor variable of node i ( t and s are variables).

I also have a travel time matrix :

int time = travelTime(int origin, int destination) 

which defines the travel time between pair of nodes. I have defined the constraint as below:

for (int i : set){
  IloIntExpr expr = model.linearIntExpr();
  expr = model.sum(model.element(t, s[i]), model.prod(t[i], -1));
  IloIntExpr next = model.element(range(0, nbNodes), s[i]);
  model.addGe(expr, travelTime(i, next));
}

***range function generates [0,1, ... ,nbNodes] and [size(s) = size(t) = nbNodes]

now the problem is that next is returning IloIntExpr and I want it to be int to pass it to travelTime(int, int), and also IloIntExpr is not castable to int. What can be the possible solution for my problem. I really appreciate any help in this regard.