int sa2a(int n)
{
    assert(n > 0);

    int nn = (n / 100) + 99-(n % 100);
    if (nn == 0)
        nn = 1;
    return f1(nn, 4);
}

thread t0(sa2a, 10);

auto future = std::async(std::launch::async, &sa2a, 10);
// do some other stuff
int result = future.get(); // wait on async completion and get result