#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 1e5 + 5;
const int M = 1e6;
const long long INF = (long long)(1e18);
const int inf = 1e9;
const long long MOD = 1000000007LL;
void solve() {}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  string s, t;
  cin >> s >> t;
  int n = s.size();
  int i = n - 1;
  string ans = s;
  while (i >= 0) {
    if (ans[i] == 'z') {
      ans[i] = 'a';
      i--;
      continue;
    }
    break;
  }
  ans[i]++;
  if (ans <= s || ans >= t) {
    cout << "No such string";
    return 0;
  }
  cout << ans;
  return 0;
}
