#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k;
  char s[111], t[111];
  while (~scanf("%s%s", s, t)) {
    int len = strlen(s);
    s[len - 1]++;
    for (i = len - 1; i >= 0; i--) {
      if (s[i] == 'z' + 1) {
        s[i - 1]++;
        s[i] = 'a';
      }
    }
    if (strcmp(s, t) == 0)
      puts("No such string");
    else
      printf("%s\n", s);
  }
  return 0;
}
