#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr);
  string a, b, c = "";
  cin >> a >> b;
  for (int i = 0; i < a.length(); i++) {
    if (a[i] + 1 < b[i]) {
      c += a[i] + 1;
      for (int j = i + 1; j < a.length(); j++) c += 'z';
      break;
    } else if (a[i] + 1 == b[i]) {
      bool big = 0;
      for (int j = i + 1; j < a.length(); j++) {
        if (a[j] != 'z') big = 1;
      }
      if (big) {
        c += a[i];
        for (int j = i + 1; j < a.length(); j++) c += 'z';
        break;
      } else {
        c += a[i] + 1;
        for (int j = i + 1; j < a.length(); j++) c += 'a';
        break;
      }
    } else
      c += a[i];
  }
  if (c != a && c != b)
    cout << c;
  else
    cout << "No such string";
  return 0;
}
