#include <bits/stdc++.h>
using namespace std;
char str[110], t[110];
int main() {
  cin >> str >> t;
  int length = strlen(str);
  for (int i = length - 1; i >= 0; --i) {
    if (str[i] == 'z')
      str[i] = 'a';
    else {
      str[i]++;
      break;
    }
  }
  if (strcmp(str, t))
    cout << str;
  else
    cout << "No such string";
  return 0;
}
