#include <stdio.h>
int e[27];
void read()
{
	char x;
	while (scanf("%c",&x) && (int) x>=97 && (int)x<=122)
	{
		e[(int)x-96]++;
	}
}
void solve()
{
	int i,j,ult_af;
	for (i=1; i<=26; i++)
		if (e[i] && i!=ult_af)
		{
			while(e[i])
			{
			printf("%c",i+96);
			ult_af=i;
			e[i]--;
			for (j=1; j<=26; j++)
				if (e[j] && j!=ult_af)
				{
					printf("%c",j+96);
					ult_af=j;
					e[j]--;
					break;
				}
			}
		}
}
int main()
{
	freopen("ordine.in","r",stdin);
	freopen("ordine.out","w",stdout);
	read();
	solve();
	return 0;
}