why the following code not compile

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#include <iostream>
using std::cout; // must need for using cout
using std::endl;

#include <string>
using std::string;

int main()
{
string stringa("abc");
string stringb("bbb");
cout << merge (stringa, stringb);


} // end main

string merge ( string a, string b){
string c = a + b;
return c;
}