/// Containers Start
#include <iostream>
#include <string>
#include <sstream>
#include <set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <vector>
#include <utility>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <cstdlib>
#include <iterator>
#include <algorithm>
#include <functional>
#include <random>
/// C Header Files
#include <stdio.h>
#include <cstdio>
#include <cctype>
#include <cmath>
#include <math.h>
#include <ctime>
#include <cstring>
/// Containers End

using namespace std;

/// Math Start
#define PI acos(-1.0)
#define Pi 3.141592653589793
#define EPS (1e-7)
#define INF (0x3f3f3f3f)
/// Math End

/// Extra Start
#define nn '\n'
#define pb push_back
#define SS stringstream
#define ull unsigned long long
#define mod 1000000007
#define SIZE 2000001
#define _cin                      \
    ios_base::sync_with_stdio(0); \
    cin.tie(0);
#define sz(a) int((a).size())
#define space " "
#define All(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define Ignore cin.ignore()
#define StringToInt               \
    if (!(istringstream(s) >> n)) \
        n = 0;
#define SORT(c) sort(All((c)))
#define RSORT(c) sort(rall((c)))
/// Extra End

/// Functions Start
template <class T>
T gcd(T a, T b)
{
    if (b == 0)
        return a;
    return gcd(b, a % b);
}

template <class T>
T lcm(T a, T b) { return a / gcd(a, b) * b; }

template <class T>
string converter(T n)
{
    SS x;
    x << n;
    return x.str();
}

string itos(int a)
{
    string s = to_string(a);
    return s;
}

template <class T>
T mod_pow(T x, T n)
{
    T res = 1;
    while (n > 0)
    {
        if (n & 1)
            res = res * x % mod;
        x = x * x % mod;
        n >>= 1;
    }
    return res;
}

std::vector<std::string> split(const std::string &str, char sep)
{
    std::vector<std::string> v;
    std::stringstream ss(str);
    std::string buffer;
    while (std::getline(ss, buffer, sep))
    {
        v.push_back(buffer);
    }
    return v;
}

#define rep(i, n) for (int i = 0; i < n; i++)
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define for_arr(array) for (auto &i : array)
#define FORD(i, a, b) for (int i = (a); i >= (b); i--)
#define FORA(arr) for (auto &i : arr)
#define Cin(a) cin >> a;
#define Cin2(a, b) cin >> a >> b;
#define Cin3(a, b, c) cin >> a >> b >> c;
#define Cin4(a, b, c, d) cin >> a >> b >> c >> d;
#define Cini(a) \
    int a;      \
    cin >> a;
#define Cinii(a, b) \
    int a, b;       \
    cin >> a >> b;
#define Ciniii(a, b, c) \
    int a, b, c;        \
    cin >> a >> b >> c;
#define Cins(s) \
    string s;   \
    cin >> s;
#define Cinc(c) \
    char c;     \
    cin >> c;
/// Functions End

/// Array Start
#define SET(a) memset(a, -1, sizeof a)
#define CLR(a) memset(a, 0, sizeof a)
#define MEM(a, val) memset(a, val, sizeof a)
/// Array End

/// Debug Start
#define deb(x) cout << #x << ": " << x << endl
#define deb2(x, y) cout << #x << ": " << x << '\t' << #y << ": " << y << endl
#define deb3(x, y, z) cout << #x << ": " << x << '\t' << #y << ": " << y << '\t' << #z << ": " << z << end
/// Debug End

/// TypeDef Start
typedef long long int ll;
typedef map<string, int> msi;
typedef map<int, int> mii;
typedef map<ll, ll> mll;
typedef map<char, int> mci;
typedef map<int, string> mis;
typedef pair<int, int> pii;
typedef pair<string, int> psi;
typedef pair<string, string> pss;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<pii> vii;
/// TypeDef End

int dx[] = {-1, 0, 1, 0};
int dy[] = {0, -1, 0, 1};

/**>>>>>>>>>>>>>>>>>>> END <<<<<<<<<<<<<<<<<<**/

///topcoder template

bool mysort(pair<int, int> a, pair<int, int> b)
{
    return a.second > b.second;
}

int main(void)
{
    Cins(s);
    char ans = 'a',c[s.length()];
    rep(i,s.length()) c[i] = s[i];
    sort(c,c+s.length());
    rep(i, s.length())
    {
        if(c[i]==ans){
            ans += 1;
        }
    }
    if(ans<='z')
        cout << char(ans) << nn;
    else
        cout << "None" << nn;
    return 0;
}


Measured memory_bytes: 512