#include <bits/stdc++.h>
using namespace std;
const int inf = 1 << 30;
const long long INF = 1LL << 60;
const int maxn = 110;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string second, t;
  cin >> second >> t;
  int n = (int)(second).size();
  for (int i = n - 1; i >= 0; i--) {
    if (second[i] != 'z') {
      second[i]++;
      break;
    } else {
      second[i] = 'a';
    }
  }
  if (second != t) {
    cout << second;
  } else {
    cout << "No such string\n";
  }
  return 0;
}
