#include <iostream>
using namespace std;
int main() {
    string s = "Hello world";
    
    s.append("welcome");
    
    cout<<s<<endl;
    
    s = s+"Arun";
    cout<<s.front()<<endl;
    cout<<s.back()<<endl;
    cout<<s.push_back("AAA");
    return 0;
}