#include<stdio.h>
#include<string.h>
int main(){
	int i,j,v[50],ok;
	char s[100000],c[26]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','r','q','s','t','u','v','w','x','y','z'};
	freopen("ordine.in","r",stdin);
	freopen("ordine.out","w",stdout);
	scanf("%s",s);
	for(i=0;i<strlen(s);++i)
		v[s[i]-'a'+1]++;
	for(i=1;i<=26;++i)
		while(v[i]){
			printf("%s",c[i]);
			v[i]--;
			ok=1;
			for(j=i+1;j<=26&&ok;++j)
				if(v[j]){
					ok=0;
					printf("%s",c[i]);
					--v[j];
				}
		}
	fclose(stdin);
	fclose(stdout);
	return 0;
}