How the answer is 0

What will be the output of following C++ code?

#include <bits/stdc++.h>
using namespace std;
int main()
{
sets;
pair<set::iterator, bool> ret;
s.insert(1);
s.insert(5);
s.insert(10);
s.insert(3);
ret = s.insert(10);
cout<<ret.second<<endl;
return 0;
}

This code is meant to show that the insert function in the set returns a pair that contains set:iterator and the value bool. The insert function is used to add new elements to the set. We have just depicted that the return value of set is a pair of set:iterator pointing to the element and value of element we just inserted.

For more info about set::insert function. You can check http://www.cplusplus.com/reference/set/set/insert/