#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int i, j;
  for (int i = 0; i < s.size(); i += 2) {
    for (int j = 0; j < s.size() - 1; j += 2) {
      if (s[j] > s[j + 2]) {
        swap(s[j], s[j + 2]);
      }
    }
  }
  cout << s << endl;
}
