main.cpp: In function ‘int main(int, char**)’:
main.cpp:66:16: error: cannot convert ‘std::string {aka std::basic_string<char>}’ to ‘const char*’ for argument ‘1’ to ‘int atoi(const char*)’
  int n = atoi(k);


 string userinput;
 string v;
 cin >> userinput;
 getline(cin, userinput);
 string endline;
 cin >> endline;
 getline(cin, endline);
 v = userinput.substr(0,1);
 string k = userinput.substr(2,' ');
 int userinputint= intconversion(v);
 int n = atoi(k);