View Compilation Error

0_0_29397143_19480.cpp: In function 'int main()':
0_0_29397143_19480.cpp:26:91: error: invalid initialization of non-const reference of type 'std::string& {aka std::basic_string<char>&}' from an rvalue of type 'std::string {aka std::basic_string<char>}'
  for (int i = 5; i < maxn; i++) f[i] = add(f[i - 1], add(f[i - 2], add(f[i - 3], f[i - 4])));
                                                                                           ^
0_0_29397143_19480.cpp:9:8: note: in passing argument 2 of 'std::string add(std::string&, std::string&)'
 string add(string &a, string &b) {
        ^