#include <bits/stdc++.h>
using namespace std;
int n, m;
char c[200];
string s, t;
void solve() {
  int i, j, x, r, l;
  s = c;
  gets(c);
  t = c;
  n = s.size();
  m = t.size();
  for (i = n - 1; i >= 0; i--) {
    if (s[i] != 'z') {
      s[i] = s[i] + 1;
      i = -1;
    } else {
      s[i] = 'a';
    }
  }
  if (s != t) {
    printf("%s", s.c_str());
  } else {
    printf("No such string");
  }
  return;
}
int main() {
  gets(c);
  solve();
  return 0;
}
