#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[101];
  char t[101];
  scanf("%s%s", s, t);
  int a = strlen(s), flag = 0, j, i;
  for (i = a - 1; i >= 0; i--) {
    if (s[i] != 'z') {
      s[i] = s[i] + 1;
      for (j = i + 1; j < a; j++) {
        s[j] = 'a';
      }
      break;
    }
  }
  if (strcmp(s, t) == 0) {
    printf("No such string\n");
  } else {
    printf("%s\n", s);
  }
  return 0;
}
