#include <bits/stdc++.h>
using namespace std;
int conversion(string p) {
  int o;
  o = atoi(p.c_str());
  return o;
}
string toString(int h) {
  stringstream ss;
  ss << h;
  return ss.str();
}
long long gcd(long long a, long long b) { return (b == 0 ? a : gcd(b, a % b)); }
long long lcm(long long a, long long b) { return (a * (b / gcd(a, b))); }
long long toi(string p) {
  long long x;
  istringstream in(p);
  in >> x;
  return x;
}
int main() {
  string a, b, x;
  cin >> a >> b;
  int cont = 0;
  for (int i = a.size() - 1; i >= 0; i--) {
    if (a[i] != 'z') {
      x = a.substr(0, i) + char(a[i] + 1) + a.substr(i + 1);
      if (x < b) {
        cout << x << endl;
        return 0;
      }
    } else
      a[i] = 'a';
  }
  cout << "No such string" << endl;
}
