how to print/access the 1st element of the first set of the vector

Code:

#include <iostream>
#include <vector>
#include <set>
using namespace std;
void add123(vector<set<int> >& numbers);

int main()
{
        vector<set<int> > vector_set;
        add123(vector_set);
        //cout << vector_set[0]; //how to print/access the 1st element of the first set of the vector vector_set?
        return 0;
}
void add123(vector<set<int> >& numbers)
{
        set<int> s;
        s.insert(123);
        numbers.push_back(s);
}

vector_set[0] is the first set. Since the expression vector_set[0] is a set object, use set's interface to access elements of the set.