#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[150], t[150];
  int n, i;
  while (cin >> s >> t) {
    n = strlen(s);
    for (i = n - 1; i >= 0; i--) {
      if (s[i] < 'z') {
        s[i]++;
        break;
      } else
        s[i] = 'a';
    }
    int x = strcmp(s, t);
    if (x < 0)
      cout << s << endl;
    else
      cout << "No such string" << endl;
  }
}
