运行下面的代码，将出现（ ）。
double hmean(double a, double b) { if (a == -b ) throw runtime_error("Runtime error occurred."); return 2.0*a*b/(a + b); } int main() { double x = 10; double y = -10; try { int result = hmean(x, y); cout << "hmean: " << result << endl; } catch (const runtime_error& e) { cout << "Caught: " << e.what() << endl; } catch (...) { cout << "Caught an unknown exception." << endl; } return 0; }