#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  string s, s1, s2 = "";
  cin >> s >> s1;
  long long int siz = s.size(), i, j;
  for (i = siz - 1; i >= 0; i--) {
    if (s[i] != 'z') break;
  }
  for (j = 0; j <= i; j++) {
    if (j == i) {
      s[j]++;
      s2 += s[j];
    } else
      s2 += s[j];
  }
  for (j = i + 1; j < siz; j++) {
    s2 += 'a';
  }
  if (s2 < s1)
    cout << s2;
  else
    cout << "No such string";
  return 0;
}
