#include <fstream>
#include <string>

using namespace std;

int a[300]  , n;
string sir;

int main()
{
	ifstream in("ordine.in");
	ofstream out("ordine.out");

	in >> sir;

	for(unsigned int i = 0 ; i < sir.length() ; i++)
	{
		a[sir[i]]++;
	}

	for(int x = 'a' ; x <= 'z' ; x++)
	{
		while(a[x] != 0)
		{
			out << (char)x;
			a[x] --;
			if(a[x] != 0)
			{
				for(int j = x + 1 ; j<= 'z' ; j++)
				{
					if(a[j] != 0)
					{
						out << (char)j;
						a[j]--;
						break;
					}
				}
			}
		}
	}
	return 0;
}