#include <bits/stdc++.h>
using namespace std;
int gcd(int a, int b) { return (b == 0 ? a : gcd(b, a % b)); }
int main() {
  ios_base::sync_with_stdio(0);
  string s, t;
  while (cin >> s >> t) {
    bool check = false;
    for (int i = 0; i < (int)s.size(); i++) {
      if (s[i] > t[i]) break;
      if (t[i] == s[i] + 1) {
        for (int j = i + 1; j < (int)s.size(); j++) {
          if (s[j] != 'z') {
            check = true;
            s[j] += 1;
            break;
          }
        }
        if (!check) {
          for (int j = i + 1; j < (int)s.size(); j++) {
            if (t[j] != 'a') {
              check = true;
              t[j] -= 1;
              s = t;
              break;
            }
          }
        }
        break;
      }
      if (t[i] - s[i] > 1) {
        check = true;
        s[i] += 1;
        break;
      }
    }
    if (check)
      cout << s << endl;
    else
      cout << "No such string" << endl;
  }
  return 0;
}
