#include <bits/stdc++.h>
using namespace std;
char a[101], b[101];
int main() {
  cin >> a >> b;
  a[strlen(a) - 1]++;
  for (int c = strlen(a) - 1; a[c] > 122;) {
    a[c] = 97;
    a[--c]++;
  }
  if (strcmp(a, b) < 0) {
    cout << a;
  } else {
    cout << "No such string";
  }
  return 1 - 1;
}
