#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T gcd(T a, T b) {
  while (b) b ^= a ^= b ^= a %= b;
  return a;
}
template <class T>
inline T lcm(T a, T b) {
  a = abs(a);
  b = abs(b);
  return (a / gcd(a, b)) * b;
}
template <class T>
inline T my_strrev(T array) {
  int N = strlen(array);
  for (int i = 0; i < N / 2; ++i) swap(array[i], array[N - i - 1]);
}
template <class T>
inline T my_revara(T array, int N) {
  for (int i = 0; i < N / 2; ++i) swap(array[i], array[N - i - 1]);
}
int fx[8] = {0, -1, 1, 1, 1, 0, -1, -1};
int fy[8] = {1, 0, 1, -1, 0, -1, -1, 1};
int main() {
  string str1, str2;
  cin >> str1;
  cin >> str2;
  int len = str1.length();
  int flag = 0;
  for (int i = 0; i < len; i++) {
    if (flag == 1) {
      if (str2[i] != 'a') {
        str2[i] = str2[i] - 1;
        cout << str2 << endl;
        return 0;
      }
      if (str1[i] != 'z') {
        str1[i] = str1[i] + 1;
        cout << str1 << endl;
        return 0;
      }
    }
    if (str2[i] > str1[i]) {
      if (str2[i] - str1[i] > 1) {
        str1[i] = str1[i] + 1;
        cout << str1 << endl;
        return 0;
      }
      flag = 1;
    }
  }
  printf("No such string\n");
  return 0;
}
