Hi All. I am trying to use Futures, but not getting the expected result. The code below works some times, and other times it throws exception

"terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::promise_already_satisfied> >'
what(): Promise already satisfied"

Any help will be appreciated.
Thanks Kazz.

//-------------------------------------------//

boost::promise<int> pt;    // holds final calculated result

void worker1()
{
    // do any processing using algorithm A
    srand ( time ( NULL) );
    int delay = rand() % 5 + 1;
    boost::this_thread::sleep( boost::posix_time::seconds(delay));

    // result calculate
    pt.set_value(1);
}

void worker2()
{
    // do any processing using algorithm B
    srand ( time ( NULL) );
    int delay = rand() % 5 + 1;
    boost::this_thread::sleep( boost::posix_time::seconds(delay));

    // result calculated
    pt.set_value(2);
}

int main() {
    boost::shared_future<int> ans(pt.get_future());

    boost::thread th1(worker1);
    boost::thread th2(worker2);

    ans.wait();

    std::cout<<ans.get()<<std::endl;

    std::cout<<"exiting"<<std::endl;

    return 0;
}