View Compilation Error

365341_0_17236.cpp: In function `int main()':
365341_0_17236.cpp:35: error: invalid initialization of non-const reference of type 'std::string&' from a temporary of type 'std::string'
365341_0_17236.cpp:9: error: in passing argument 2 of `std::string hp_mul(std::string&, std::string&)'