#include <bits/stdc++.h>
using namespace std;
string s1, s2;
int main() {
  cin >> s1 >> s2;
  int i = 0;
  for (; i < s1.size() && s1[i] == s2[i]; i++)
    ;
  if (i == s1.size() - 1) {
    if (s2[i] - s1[i] == 1)
      cout << "No such string";
    else {
      s1[i]++;
      cout << s1;
    }
  } else {
    int y = i++;
    for (; i < s1.size(); i++)
      if (s1[i] != 'z') {
        s1[i]++;
        cout << s1;
        return 0;
      }
    s1[y]++;
    for (i = y + 1; i < s1.size(); i++) s1[i] = 'a';
    for (i = y; i < s1.size() && s1[i] == s2[i]; i++)
      ;
    if (i == s1.size())
      cout << "No such string";
    else
      cout << s1;
  }
  return 0;
}
