#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b, c;
  cin >> a >> b;
  int len = a.size();
  c.resize(len);
  for (int i = 0; i < len; i++) {
    if (b[i] != a[i]) {
      if (b[i] - a[i] > 1)
        c[i] = a[i] + 1;
      else
        c[i] = b[i];
      for (int j = i + 1; j < len; j++) {
        c[j] = 'a';
      }
      if (c < b && c > a) {
        cout << c << endl;
        return 0;
      }
      c[i] = a[i];
      for (int j = i + 1; j < len; j++) {
        c[j] = 'z';
      }
      if (c < b && c > a) {
        cout << c << endl;
        return 0;
      }
    } else
      c[i] = b[i];
  }
  cout << "No such string\n";
  return 0;
}
