#include <bits/stdc++.h>
using namespace std;
string inc(string s, int p) {
  if (p == -1) {
    s = 'a' + s;
    return s;
  }
  if (s[p] < 'z') {
    s[p]++;
  } else {
    s[p] = 'a';
    s = inc(s, p - 1);
  }
  return s;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s, t;
  cin >> s >> t;
  s = inc(s, int(s.size()) - 1);
  if (s < t)
    cout << s << endl;
  else
    cout << "No such string" << endl;
  return 0;
}
