#include <bits/stdc++.h>
using namespace std;
const long long int dx[4] = {+1, -1, 0, 0};
const long long int dy[4] = {0, 0, +1, -1};
const long long int MOD = 1e9 + 7;
const long long int INF = 1e10;
string s, t, ans;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> s >> t;
  string ans = s;
  for (long long int i = s.length() - 1; i >= 0; i--) {
    if (s[i] != 'z') {
      ans[i]++;
      break;
    } else {
      ans[i] = 'a';
    }
  }
  if (ans != t)
    cout << ans;
  else
    cout << "No such string";
  return 0;
}
