#include<stdio.h>
#include<string.h>
#define N 1000001
int main(){
	int i,j,v[27],ok;
	char s[N];
	freopen("ordine.in","r",stdin);
	freopen("ordine.out","w",stdout);
	scanf("%s",s);
	for(i=1;i<=27;++i)
		v[i]=0;
	for(i=0;i<strlen(s);++i)
		v[s[i]-'a'+1]++;
	for(i=1;i<=27;++i)
		while(v[i]){
			printf("%c",('a'-1+i));
			v[i]--;
			if(v[i]){
				ok=1;
				for(j=i+1;j<=27 && ok;++j)
					if(v[j]){
						ok=0;
						printf("%c",('a'-1+j));
						--v[j];
					}
			}
		}
	printf("\n");
	fclose(stdin);
	fclose(stdout);
	return 0;
}