#include <bits/stdc++.h>
using namespace std;
int a[100001] = {0}, l[100001] = {0}, pos[100001] = {0};
int res[100] = {0};
int main() {
  string s1, s2;
  cin >> s1 >> s2;
  int k = s1.length();
  s1[k - 1]++;
  for (int i = k - 1; i >= 1; i--) {
    if (s1[i] > 'z') {
      s1[i] -= 26;
      s1[i - 1]++;
    }
  }
  if (s1 == s2)
    cout << "No such string" << endl;
  else
    cout << s1 << endl;
}
