#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <queue>
#include <functional>
#include <map>
#include <set>
#include <sys/timeb.h>
#include <fstream>
#include <random>
#include <regex>
#include <chrono>
#include <bitset>
#include <cassert>
#include <iomanip>
#include <limits>
#include <stack>

using namespace std;

#define DEBUG_

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define reprrev(i, a, b) for (int i = (int)(b)-1; i >= (int)(a); i--)
#define reprev(i, n) reprrev(i, 0, n)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define mp make_pair
#define mt make_tuple
#define INF 1050000000
#define INFR INT_MAX
#define INFL (long long)(4e18)
#define INFLR LLONG_MAX
#define EPS (1e-10)
#define MOD 1000000007
//#define MOD 998244353
#define PI 3.141592653589793238
#define RMAX 4294967295

#ifdef DEBUG_
#define DEB
#else
#define DEB if (false)
#endif

#ifdef DEBUG_
#define dump(...)                                                                                \
	DUMPOUT << "  ";                                                                             \
	dump_func(string(#__VA_ARGS__) + ":", "[" + to_string(__LINE__) + ":" + __FUNCTION__ + "]"); \
	DUMPOUT << "    ";                                                                           \
	dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

typedef long long ll;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<Pi> vPi;
typedef vector<Pll> vPll;
typedef vector<Pd> vPd;
typedef vector<vector<int>> vvi;
typedef vector<vector<bool>> vvb;
typedef vector<vector<ll>> vvll;
typedef vector<vector<char>> vvc;
typedef vector<vector<string>> vvs;
typedef vector<vector<double>> vvd;
typedef vector<vector<Pi>> vvPi;
typedef vector<vector<vector<int>>> vvvi;
typedef vector<vector<vector<ll>>> vvvll;
typedef vector<vector<vector<Pi>>> vvvPi;
typedef vector<vector<vector<vector<Pi>>>> vvvvPi;
typedef priority_queue<int, vector<int>, greater<int>> pqli;
typedef priority_queue<ll, vector<ll>, greater<ll>> pqlll;
typedef priority_queue<Pi, vector<Pi>, greater<Pi>> pqlP;
template <class T>
using vec = vector<T>;
template <class T>
using pql = priority_queue<T, vector<T>, greater<T>>;
string debug_show(Pi a)
{
	return "(" + to_string(a.first) + "," + to_string(a.second) + ")";
}

#define DUMPOUT cerr

void dump_func()
{
	DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&... tail)
{
	DUMPOUT << head;
	if (sizeof...(Tail) <= 1)
	{
		DUMPOUT << " ";
	}
	else
	{
		DUMPOUT << ", ";
	}
	dump_func(std::move(tail)...);
}

// vector
template <typename T>
istream &operator>>(istream &is, vector<T> &vec)
{
	for (T &x : vec)
		is >> x;
	return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var)
{
	os << "(" << pair_var.first << "," << pair_var.second << ")";
	return os;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &pair_var)
{
	os << "(" << pair_var.first << "," << pair_var.second << ")";
	return os;
}
// vector
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec)
{
	os << "{";
	for (int i = 0; i < vec.size(); i++)
	{
		os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
	}
	os << "}";
	return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var)
{
	os << "{";
	repi(itr, map_var)
	{
		os << "(" << itr->first << "," << itr->second << ")";
		itr++;
		if (itr == map_var.end())
			os << ", ";
		itr--;
	}
	os << "}";
	return os;
}
// set
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var)
{
	os << "{";
	repi(itr, set_var)
	{
		os << "  " << *itr << endl;
		itr++;
		if (itr == set_var.end())
			os << ", ";
		itr--;
	}
	os << "}";
	return os;
}
string YN(bool y, int id = 0)
{
	if (id)
		cout << id;
	return (y ? "YES" : "NO");
}
string yn(bool y, int id = 0)
{
	if (id)
		cout << id;
	return (y ? "Yes" : "No");
}
string ON(bool y, int id = 0)
{
	if (id)
		cout << id;
	return (y ? "OK" : "NG");
}

int dir4[4][2] = {{0, -1}, {-1, 0}, {1, 0}, {0, 1}};
int dir8[8][2] = {{-1, -1}, {0, -1}, {1, -1}, {-1, 0}, {1, 0}, {-1, 1}, {0, 1}, {1, 1}};
char dirchar[4] = {'<', '^', '>', 'v'};

// [a,b)
int irand(int a, int b)
{
	static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
	uniform_int_distribution<int> dist(a, b - 1);
	return dist(Rand);
}

// [a,b)
double drand(int a, int b)
{
	static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
	uniform_real_distribution<double> dist(a, b);
	return dist(Rand);
}

template <typename T>
T max(vector<T> a)
{
	return *max_element(all(a));
}
template <typename T>
T min(vector<T> a)
{
	return *min_element(all(a));
}
template <typename T>
T sum(vector<T> a)
{
	return accumulate(all(a), 0);
}

struct Edge
{
	int from, to, cost;
	bool operator<(Edge e)
	{
		return cost < e.cost;
	}
};
using Graph = vec<vec<Edge>>;

ostream &operator<<(ostream &os, Edge &edge)
{
	os << "(" << edge.from << "->" << edge.to << ":" << edge.cost << ")";
	return os;
}

//======================================================
int main(void)
{
	string s;
	cin >> s;
	vi num(26, 0);
	for (auto c : s)
	{
		num[c - 'a']++;
	}
	rep(i, 26)
	{
		if (num[i] == 0)
		{
			cout << (char)(i + 'a') << endl;
			return 0;
		}
	}
	cout << "None" << endl;
	return 0;
}

Measured memory_bytes: 512