#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s1, s2;
  int i, j, l;
  cin >> s1 >> s2;
  l = s1.length();
  bool ch = false;
  for (i = l - 1; i >= 0; i--) {
    if (s1[i] != 'z') {
      s1[i]++;
      for (j = i + 1; j < l; j++) s1[j] = 'a';
      ch = true;
      break;
    }
  }
  if (ch and s1 < s2)
    cout << s1 << endl;
  else
    cout << "No such string\n";
  return 0;
}
