#include <bits/stdc++.h>
using namespace std;
int GCD(int x, int y) { return !y ? x : GCD(y, x % y); }
int LCM(int x, int y) { return x * y / GCD(x, y); }
long long ceilDivision(long long dividend, long long divisor) {
  return (dividend + divisor - 1) / divisor;
}
string Add1(string c) {
  string a = c;
  for (int i = a.size() - 1; i >= 0; i--) {
    if (a[i] < 'z') {
      a[i]++;
      return a;
    } else {
      while (i >= 0 && a[i] == 'z') a[i] = 'a', i--;
      if (i < 0)
        return c;
      else {
        a[i]++;
        return a;
      }
    }
  }
}
int main() {
  string a, b, c;
  cin >> a >> b;
  c = Add1(a);
  if (c >= b || c <= a)
    cout << "No such string" << endl;
  else
    cout << c << endl;
  return 0;
}
