#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 10;
char a[N];
map<string, int> mp;
int st[N];
string s1;
string s2;
int main() {
  cin >> s1 >> s2;
  int n = s1.length() - 1;
  s1[n]++;
  if (s1 >= s2) {
    cout << "No such string" << endl;
  } else {
    while (s1[n] == 'z' + 1) {
      s1[n] = 'a';
      s1[--n]++;
    }
    if (s1 >= s2) {
      cout << "No such string" << endl;
    } else {
      cout << s1 << endl;
    }
  }
}
