#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[101], t[101];
  cin >> s;
  cin >> t;
  int n = strlen(s);
  int aux1;
  for (int i = n - 1; i >= 0; i--) {
    if (s[i] == 'z') {
      s[i] = 'a';
    } else {
      s[i] = s[i] + 1;
      break;
    }
  }
  if (strcmp(s, t) == -1) {
    cout << s << endl;
  } else {
    cout << "No such string" << endl;
  }
}
