#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b;
  cin >> a >> b;
  int carry = 1;
  for (int i = a.size() - 1; i >= 0; i--) {
    a[i] += 1;
    if (a[i] > 'z') {
      carry = 1;
      a[i] = 'a';
    } else {
      break;
    }
  }
  if (a == b) {
    cout << "No such string";
  } else {
    cout << a;
  }
  return 0;
}
