#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b, a1;
  cin >> a;
  cin >> b;
  a1 = a;
  int i = a.length() - 1;
  if (a == b) {
    cout << "No such string";
    return 0;
  }
  while (i >= 0) {
    if (a[i] == 'z') {
      i--;
      continue;
    }
    a[i] = a[i] + 1;
    break;
  }
  i = a.length() - 1;
  int j;
  for (j = 0; j <= i; j++) {
    if (a[j] > a1[j]) break;
  }
  j++;
  for (int z = j; z <= i; z++) {
    if (a[z] < b[z]) continue;
    if (b[z] == 'a') a[z] = 'a';
    if (b[z] != 'a') {
      a[z] = b[z] - 1;
    }
  }
  if (a == a1 || a == b)
    cout << "No such string";
  else
    cout << a;
}
