#include <bits/stdc++.h>
using namespace std;
char a[102];
char b[102];
char c[102];
int main() {
  gets(a);
  gets(b);
  int l = strlen(a);
  int i;
  for (i = l - 1; i >= 0; i--) {
    if (a[i] == 'z') {
      a[i] = 'a';
    } else {
      a[i] = a[i] + 1;
      break;
    }
  }
  if (!strcmp(a, b))
    cout << "No such string";
  else
    cout << a;
  return 0;
}
