#include <stdio.h>
#include <string>
#define FOR(i,s,d) for(i=(s);i<=(d);++i)
char c[1000005],sir[1000005];
long v[30],n,i,j,x,ok;
int main()
{
    freopen ("ordine.in","r",stdin);
    freopen ("ordine.out","w",stdout);
    scanf("%s",&c);
    x=strlen(c);
    FOR (i,0,x-1)v[c[i]-96]++;
    FOR (i,1,x){
        ok=1;
        FOR (j,1,26)
            if (v[j]==(x-i+1)/2+1&&j+96!=sir[i-1])
               {sir[i]=j+96;v[j]--;ok=0;break;}
        if(ok!=0)
        FOR (j,1,26)
            if (j+96!=sir[i-1]&&v[j]>0)
               {sir[i]=j+96;v[j]--;break;}
        }
    FOR(i,1,x)
    printf("%c",sir[i]);
    /*printf("\n%ld",x);
    c[2]=' ';
    printf("%ld",c[2]);*/
    return 0;
}