#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("unroll-loops")
using namespace std;
long long ta, n, m, a, b, c, d, x, y, sum = 0, cnt, k, tot = 0, q, w, u, s, e,
                                      h, tb;
vector<long long> v;
map<long long, long long> mp;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s, t;
  cin >> s >> t;
  string ans, ans2;
  long long l = s.size();
  long long posi = -1;
  bool flag = 0;
  for (long long i = 0; i < l; i++) {
    if (s[i] != t[i]) {
      char ch = s[i];
      ch++;
      if (t[i] != ch) {
        ans += ch;
        flag = 1;
        posi = i + 1;
        break;
      } else {
        ans += t[i];
        posi = i + 1;
        break;
      }
    } else
      ans += t[i];
  }
  for (long long i = posi; i < l; i++) {
    if (t[i] > 'a') {
      ans += 'a';
      flag = 1;
    } else
      ans += 'a';
  }
  if (!flag) {
    for (long long i = l - 1; i >= posi and posi != -1; i--) {
      if (s[i] != 'z') {
        char ch = s[i];
        ch++;
        flag = 1;
        ans[i] = ch;
        ans[posi - 1] = s[posi - 1];
        break;
      }
    }
  }
  if (flag)
    cout << ans << endl;
  else
    cout << "No such string" << endl;
  return 0;
}
