#include <bits/stdc++.h>
using namespace std;
int main() {
  string answer = "No such string";
  string a, b;
  bool allz = true;
  int q;
  cin >> a;
  cin >> b;
  for (int i = a.size() - 1; i >= 0; i--) {
    if (a[i] != 'z') {
      a[i]++;
      allz = false;
      for (int j = i + 1; j < a.size(); j++) a[j] = 'a';
      break;
    }
  }
  if (allz)
    cout << answer;
  else {
    for (int i = 0; i < a.size(); i++) {
      if (b[i] > a[i]) {
        answer = a;
        break;
      }
    }
    cout << answer;
    return 0;
  }
}
