#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[110], t[110], a[110];
  cin >> s;
  cin >> t;
  strcpy(a, t);
  int i, l;
  l = strlen(a);
  for (i = l - 1; i >= 0; i--) {
    if (a[i] - 1 >= 'a') {
      a[i]--;
      if (strcmp(s, a) >= 0 && i < l - 1) {
        a[i + 1]++;
      }
      break;
    }
  }
  if (strcmp(s, a) < 0)
    cout << a << endl;
  else
    cout << "No such string" << endl;
  return 0;
}
