#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
using ii = pair<long long int, long long int>;
class prioritize {
 public:
  bool operator()(ii &p1, ii &p2) { return p1.first < p2.first; }
};
auto start_time = high_resolution_clock::now();
void time() { return; }
void ojudge() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  return;
}
int main() {
  ojudge();
  string s, t;
  cin >> s >> t;
  long long int n = (long long int)s.size();
  if (n == 1) {
    if (abs(t[0] - s[0]) <= 1)
      cout << "No such string\n";
    else {
      cout << char(s[0] + 1) << "\n";
    }
    return 0;
  }
  long long int cnt = 0;
  long long int first = 0;
  long long int idy = 0;
  for (long long int i = 0; i < n; i++) {
    if (s[i] != t[i]) cnt++;
    if (t[i] - s[i] > 1) {
      first++;
      idy = i;
    }
  }
  if (first) {
    t[idy] = char(t[idy] - 1);
    cout << t << "\n";
    return 0;
  }
  if (cnt == 0)
    cout << "No such string\n";
  else {
    long long int id = 0;
    string res = "";
    while (id < n) {
      if (s[id] == t[id])
        id++;
      else
        break;
    }
    string resu = "";
    if (id == n - 1)
      cout << "No such string\n";
    else {
      for (long long int i = 0; i < id + 1; i++) {
        resu.push_back(t[i]);
      }
      bool is = false;
      for (long long int j = id + 1; j < n; j++) {
        if (t[j] > 'a') {
          is = true;
          resu.push_back('a');
        } else
          resu.push_back(t[j]);
      }
      if (is == false) {
        string anss = "";
        bool isi = false;
        for (long long int i = 0; i < id + 1; i++) {
          anss.push_back(s[i]);
        }
        for (long long int j = id + 1; j < n; j++) {
          if (s[j] < 'z') {
            isi = true;
            anss.push_back('z');
          } else
            anss.push_back(s[j]);
        }
        if (isi)
          cout << anss << "\n";
        else
          cout << "No such string\n";
      } else
        cout << resu << "\n";
    }
  }
  time();
  return 0;
}
