#include <bits/stdc++.h>
using namespace std;
char s[1000], t[1000];
int main() {
  cin >> s >> t;
  int a, b;
  a = b = 0;
  for (int i = 0; s[i]; i++) {
    a = a * 26 + s[i] - 'a';
    b = b * 26 + t[i] - 'a';
  }
  if (a + 1 == b)
    cout << "No such string" << endl;
  else {
    int l = strlen(s);
    s[l - 1]++;
    for (int i = l - 1; i >= 0; i--)
      if (s[i] == 'z' + 1) {
        s[i - 1]++;
        s[i] = 'a';
      }
    cout << s << endl;
  }
  return 0;
}
