#include <bits/stdc++.h>
using namespace std;
string s, t;
int i, j, k, l;
int main() {
  cin >> s >> t;
  for (i = 0; i < s.length(); i++) {
    if (s[i] != t[i]) {
      i = (t[i] - s[i] > 1) ? i : i + 1;
      break;
    }
  }
  j = i;
  while (i < s.length()) {
    if (t[i] != 'a') {
      t[i]--;
      return cout << t, 0;
    }
    i++;
  }
  i = j;
  while (i < s.length()) {
    if (s[i] != 'z') {
      s[i]++;
      return cout << s, 0;
    }
    i++;
  }
  cout << "No such string";
}
