#include <fstream>
using namespace std;
ifstream f ("ordine.in");
ofstream g ("ordine.out");
char j,c,ant=0,s[1000003];
int n,i,ok,ap[123];
int main()
{
    f>>s;
    for(n=0;s[n];++n) ++ap[s[n]];
    for(i=0;i<n;++i)
    {
        for(ok=1,j='a';j<='z'&&ok;++j) if(ap[j]==(n-i)/2+1) {c=j;ok=0;}
        for(j='a';j<='z'&&ok;++j) if(ap[j]&&j!=ant) {c=j;ok=0;}
        g<<c;
        --ap[c];
        ant=c;
    }
    g<<'\n';
    return 0;
}