#include <cstdio>
#include <string>
#include <deque>
using namespace std;

int letters[200] = { 0 };

int main() {
	char input[1000010] = { '\0' };
	scanf("%s", input);
	for (int i = 0; input[i] != '\0'; i++) {
		letters[(int)input[i]]++;
	}
	deque<char> out;
	int odd = -1;
	for (int i = 0; i < 200; i++) {
		if (letters[i] % 2 == 1) {
			if (odd < 0) odd = i;
			else {
				printf("IMPOSSIBLE");
				return 0;
			}
		}
	}
	if (odd > 0) {
		out.push_back((char)odd);
		letters[odd]--; 
	}
	for (int i = 199; i >= 0; i--) {
		for (int j = letters[i] / 2; j < letters[i]; j++) {
			out.push_front((char)i);
			out.push_back((char)i);
		}
	}
	for (char& ch : out) {
		printf("%c", ch);
	}
}