#include <bits/stdc++.h>
using namespace std;
int main() {
  char a[1000], b[1000];
  int i, flag = 0;
  while (cin >> a >> b) {
    int n;
    n = strlen(a);
    for (i = n - 1; i >= 0; i--) {
      if (a[i] != 'z') {
        a[i] = a[i] + 1;
        break;
      } else
        a[i] = 'a';
    }
    for (i = 0; i < n; i++) {
      if (a[i] == b[i]) flag++;
    }
    if (flag != n)
      printf("%s\n", a);
    else
      printf("No such string\n");
    flag = 0;
  }
}
