#include <bits/stdc++.h>
char a[101], b[101];
int main() {
  scanf("%s\n%s", &a, &b);
  int n = strlen(a);
  a[n - 1]++;
  for (int i = n - 1; i > 0; --i) {
    if (a[i] > 'z') a[i] = 'a', a[i - 1]++;
  }
  if (strcmp(a, b) == 0)
    puts("No such string");
  else
    puts(a);
  return 0;
}
