#include <bits/stdc++.h>
using namespace std;
int main() {
  vector<string> v;
  string s, mid, t;
  cin >> s >> t;
  bool flag = false;
  for (int i = s.length() - 1; i >= 0; i--) {
    if (s[i] != 'z') {
      flag = true;
    }
  }
  if (s == t) flag = false;
  if (!flag) {
    cout << "No such string";
  } else {
    for (int i = s.length() - 1; i >= 0; i--) {
      if (s[i] == 'z')
        s[i] = 'a';
      else {
        s[i] = (char)(s[i] + 1);
        break;
      }
    }
    if (s < t) {
      cout << s;
    } else {
      printf("No such string");
    }
  }
}
