#include <bits/stdc++.h>
using namespace std;
const long long N = 1e6 + 2;
const long long MOD = 1e9 + 7;
const long long Inf = MOD * MOD;
const long double eps = 1e-7;
const long long dx[] = {1, 0, 0, 0};
const long long dy[] = {0, 1, 1, -1};
long long n, k, timer = 0, t = 1;
string s[3];
void Solve() {
  cin >> s[0] >> s[1];
  n = s[0].size();
  s[2] = s[0];
  long long Last = 1;
  for (int i = n - 1; i >= 0; --i) {
    if ((s[2][i] - 'a' + Last) >= 26) {
      s[2][i] = 'a';
    } else {
      s[2][i] = char(s[2][i] + Last);
      break;
    }
  }
  if (s[2] == s[1]) {
    cout << "No such string";
    return;
  }
  cout << s[2];
}
int main() {
  while (t--) {
    ++timer;
    Solve();
    puts("");
  }
}
