#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b, ans;
  cin >> a >> b;
  int i, l, f = 0, f1 = 0;
  ans = a;
  l = a.size();
  for (i = l - 1; i >= 0; i--) {
    if (ans[i] != 'z') {
      ans[i] += 1;
      break;
    } else
      ans[i] = 'a';
  }
  if (ans > a && ans < b)
    cout << ans;
  else
    cout << "No such string";
  return 0;
}
