#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7, MAX = 1e5 + 5;
long long powN(long long a, long long p) {
  if (p == 0) return 1;
  long long z = powN(a, p / 2);
  z = (z * z) % MOD;
  if (p % 2) z = (z * a) % MOD;
  return z;
}
vector<bool> is_prime(MAX + 1, true);
void Sieve() {
  is_prime[0] = is_prime[1] = false;
  int i, j;
  for (i = 2; i * i <= MAX; i++) {
    if (is_prime[i]) {
      for (j = i * i; j <= MAX; j += i) is_prime[j] = false;
    }
  }
}
int main() {
  string s, t;
  cin >> s >> t;
  int n = s.length();
  string s1 = s;
  if (s == t)
    cout << "No such string" << endl;
  else {
    bool ans = false;
    bool dec = false;
    for (int i = 0; i < n; i++) {
      if (dec) {
        if (t[i] == 'a') {
          s[i] = t[i];
          continue;
        }
        s[i] = (t[i] - 1);
        ans = true;
        cout << s << endl;
        break;
      } else if (t[i] - s[i] > 1) {
        ans = true;
        s[i] = t[i] - 1;
        cout << s << endl;
        break;
      } else if (t[i] - s[i] == 1) {
        s[i]++;
        dec = true;
      } else
        continue;
    }
    if (!ans) {
      dec = false;
      for (int i = 0; i < n; i++) {
        if (dec) {
          if (s1[i] == 'z') continue;
          s1[i]++;
          ans = true;
          cout << s1 << endl;
          break;
        } else if (t[i] - s1[i] > 1) {
          ans = true;
          s1[i] = t[i] - 1;
          cout << s1 << endl;
          break;
        } else if (t[i] - s1[i] == 1) {
          dec = true;
        } else
          continue;
      }
    }
    if (!ans) cout << "No such string" << endl;
  }
}
