#include <bits/stdc++.h>
using namespace std;
string a, b;
int main() {
  cin >> a >> b;
  int len = a.length();
  char end = 'z' + 1;
  int count = 1;
  while (count) {
    len--;
    if (len < 0) {
      cout << "No such string";
      return 0;
    }
    a[len]++;
    if (a[len] == end) {
      a[len] = 'a';
      count = 1;
    } else
      count = 0;
  }
  if (a == b)
    cout << "No such string";
  else
    cout << a;
}
