#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k, l, m, n, o, p;
  char s[110], s1[110];
  while (gets(s) && gets(s1)) {
    k = strlen(s);
    for (i = k; i >= 0; i--) {
      if (s[i - 1] == 'z') {
        s[i - 1] = 'a';
      } else {
        s[i - 1] = s[i - 1] + 1;
        break;
      }
    }
    if (strcmp(s, s1) == 0)
      printf("No such string\n");
    else
      printf("%s\n", s);
  }
  return 0;
}
