napaka v kodi c++

#include <iostream>	
#include <stdlib.h> 
#include <string> 
using namespace std; 



int main()
{
	string firstname, lastname;
	cout << "enter your first and last name\n";
	cin >> firstname, lastname;
	cout << "your last name is spelled:\n";
	for (int i = 0; i < lastname.length(); i++)
	{
		cout << lastname[i] << " ";
		lastname[i] = '_';
	}
	cout << endl;
	for (int i = 0; i < lastname.length(); i++)
		cout << lastname[i] << " ";


	cout << endl;


	cout << "good day " << firstname << endl;


	system("pause")
	return 0;
}