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