#include <bits/stdc++.h>
using namespace std;
inline void get_val(int &a) {
  int value = 0, s = 1;
  char c;
  while ((c = getchar()) == ' ' || c == '\n')
    ;
  if (c == '-')
    s = -s;
  else
    value = c - 48;
  while ((c = getchar()) >= '0' && c <= '9') value = value * 10 + c - 48;
  a = s * value;
}
int main() {
  string a, b, ans;
  cin >> a >> b;
  if (a >= b) {
    puts("No such string");
    return 0;
  }
  a[a.size() - 1] += 1;
  for (int i = a.size() - 1; i >= 0; --i) {
    if (a[i] == 'z' + 1) {
      a[i] = 'a';
      a[i - 1]++;
    }
  }
  if (a == b) {
    puts("No such string");
    return 0;
  }
  cout << a << endl;
  return 0;
}
