#include <bits/stdc++.h>
using namespace std;
int main() {
  char buff[110];
  int len;
  string s;
  string a;
  string t;
  scanf("%s", buff);
  s = string(buff);
  len = strlen(buff);
  buff[len - 1]++;
  for (int i = len - 1; i > 0; i--) {
    if (buff[i] > 'z') {
      buff[i] = 'a';
      buff[i - 1]++;
    }
  }
  if (buff[0] > 'z') {
    puts("No such string");
    return 0;
  }
  a = string(buff);
  scanf("%s", buff);
  t = string(buff);
  if (s < a && a < t)
    printf("%s\n", a.c_str());
  else
    puts("No such string");
  return 0;
}
