I can't understand what's wrong with my code:

#include <iostream>
#include <string>
using namespace std;

bool isPalindrom(string str, unsigned n){
    unsigned num1=n, num2=str.size()-n-1, num;
    if(num1>num2) num=num2;
    else num=num1;
    for(unsigned a=0; a<=num; a++)
        if(str[n-a]!=str[n+a])
            return false;
    return true;
    }

int main(){
    string s1;
    unsigned x;
    cin>>s1;
    if(s1.size()==1){
        cout<<s1;
        return 0;
        }
    string str;
    for(unsigned a=s1.size()/2; a<s1.size(); a++)
    if(isPalindrom(s1, a)){
        x=a;
    for(unsigned a=0; a<x; a++)
        str+=s1[a];
    str+=s1[x];
    for(int b=x-1; b>=0; b--)
        str+=s1[b];
    if(s1 < str){
    cout<<str<<endl;
    return 0;
    }
    str.erase();
   }
    return 0;
    }

1) Please, delete your code
2) Read task more carefully: "You are to find a NONEMPTY word S2 of the minimal length that S1S2 - is a palindrome."

I think your solution will fail on testcase like "SOS"