#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
int main() {
  string str, str2;
  cin >> str >> str2;
  int l = str.size();
  int i;
  string strano = "";
  for (i = l - 1; i >= 0; i--) {
    if (str[i] != 'z') {
      break;
    }
  }
  int mn;
  for (mn = 0; mn < i; mn++) {
    strano += str[mn];
  }
  if (i >= 0 && i <= (l - 1)) strano += (char)(str[i] + 1);
  for (mn = i + 1; mn < l; mn++) {
    strano += 'a';
  }
  int sz = strano.size();
  if (strano < str2 && strano > str && sz == l) {
    cout << strano << endl;
  } else
    cout << "No such string" << endl;
  return 0;
}
