#include <bits/stdc++.h>
using namespace std;
const int maxn = 111;
char s[maxn], t[maxn];
int n;
void solve() {
  for (int i = n - 1; i >= 0; i--)
    if (s[i] != 'z') {
      s[i]++;
      for (int j = i + 1; j < n; j++) s[j] = 'a';
      break;
    }
  if (!strcmp(s, t))
    puts("No such string");
  else
    puts(s);
}
int main() {
  scanf("%s%s", s, t);
  n = strlen(s);
  solve();
  return 0;
}
