#include <bits/stdc++.h>
using namespace std;
int i, j, k, l, n, m;
string s, t, a, b;
int main() {
  cin >> s >> t;
  n = t.length();
  a = t, b = s, m = s.length();
  while (n > 0 && t[n - 1] == 'a') n--;
  if (n > 0) a[n - 1]--;
  while (m > 0 && s[m - 1] == 'z') m--;
  if (m > 0) b[m - 1]++;
  if (n == 0 || m == 0 || (s >= a && b >= t))
    cout << "No such string";
  else if (s >= a)
    cout << b;
  else
    cout << a;
}
