#include <bits/stdc++.h>
using namespace std;
char v1[110], v2[110], sol[110];
int main() {
  scanf("%s\n%s", v1, v2);
  int n = strlen(v1);
  for (int i = 0; i < n; i++)
    if (v2[i] != v1[i]) {
      if (v2[i] - v1[i] > 1) {
        sol[i] = v1[i] + 1;
        for (int j = i + 1; j < n; j++) sol[j] = 'a';
        printf("%s", sol);
        return 0;
      } else {
        int k1 = 0, k2 = 0;
        for (int j = i + 1; j < n; j++)
          if (v2[j] > 'a') {
            k1 = 1;
            break;
          }
        for (int j = i + 1; j < n; j++)
          if (v1[j] < 'z') {
            k2 = 1;
            break;
          }
        if (k1) {
          sol[i] = v2[i];
          for (int j = i + 1; j < n; j++) sol[j] = 'a';
          printf("%s", sol);
          return 0;
        } else if (k2) {
          sol[i] = v1[i];
          for (int j = i + 1; j < n; j++) sol[j] = 'z';
          printf("%s", sol);
          return 0;
        } else {
          printf("No such string");
          return 0;
        }
      }
      break;
    } else
      sol[i] = v1[i];
  printf("No such string");
  return 0;
}
