#include <stdio.h>
#include <math.h>
#include <stdlib.h>

long i, o, aux, m, vb1;
int v[100010];
char ch, z;

int cmp (const void * a, const void * b) {
  return ( *(int*)a - *(int*)b );
}

int main() {
	freopen("ordine.in", "r", stdin);
	freopen("ordine.out", "w", stdout);
	while (scanf("%c", &ch) == 1) {
		v[++o] = (long)(ch) - 96;
	}
	qsort(v, o + 1, sizeof(int), cmp);
	/*for (i = 1;i <= o; ++i) {
		printf("%d ", v[i]);
	}*/
	vb1 = 1;
	while (vb1 == 1) {
		vb1 = 0;
		for (i = 2;i <= o; ++i) {
			if (v[i] == v[i - 1]) {
				aux = i + 1;
				while (v[i] == v[aux] && aux < o) {
					++aux;
				}
				if (v[i] != v[aux] && aux <= o) {
					m = v[i];
					v[i] = v[aux];
					v[aux] = m;
				} else {
					aux = i - 1;
					while (v[i] == v[aux] && aux > 1) {
						--aux;
					}
					m = v[i];
					v[i] = v[aux];
					v[aux] = m;
				}
				vb1 = 1;
			}
		}
	}
	for (i = 1;i <= o; ++i) {
		z = (char)(v[i] + 96);
		printf("%c", z);
	}
	return 0;
}