#include<fstream.h>
#include<string.h>
ifstream f("ordine.in");
ofstream g("ordine.out");
char a[1000000],b[1000000],d;
int main()
{
f>>a;
int ga=0,j,i,q=0,p;
p=strlen(a);
for(char k='a';k<='z';k++)
   for(j=0;j<=p;j++)
	   if(a[j]==k)
			b[q++]=a[j];

for(j=0;j<=p&&!ga;j++)
	{ga=1;
	 for(i=1;i<p-1;i++)
		 if(b[i]==b[i-1])
		 {d=b[i];b[i]=b[i+1];b[i+1]=d;ga=0;}
	}
ga=1;
for(j=p-1;j>=0&&ga;j--)
{ga=0;
 for(i=p-1;i>1;i--)
	 if(b[i-1]==b[i])
	 {d=b[i-1];b[i-1]=b[i-2];b[i-2]=d;ga=1;}
}
for(i=0;i<p;i++)
g<<b[i];
return 0;
}