View Compilation Error

422288_0_0_1992.cpp: In function `std::string add(std::string&, std::string&, std::string&)':
422288_0_0_1992.cpp:11: error: invalid conversion from `int' to `const char*'
422288_0_0_1992.cpp:11: error:   initializing argument 1 of `std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]'