/*
URL_HERE
*/
/*

*/
#ifdef _WIN32
#pragma warning(disable:4996)
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <algorithm>
#include <math.h>
#include <unordered_map>
using namespace std;

FILE* _fin = stdin;
FILE* _fout = stdout;
#define PI 3.141592653589793238462643383279502884197169399375105820974
#define ten5p1 100001
#define ten6p1 1000001
#define ten8p1 100000001
#define ten9p1 1000000001
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define rep(var,n) for(int var=0;var<n;++var)
#define repi(n) rep(i,n)
#define repj(n) rep(j,n)
#define repi1(n) for(int i=1;i<n;++i)
#define repj1(n) for(int j=1;j<n;++j)
int _min(int a, int b) { return a <= b ? a : b; }
int _min(ll a, ll b) { return a <= b ? a : b; }
int _max(int a, int b) { return a >= b ? a : b; }
int _max(ll a, ll b) { return a >= b ? a : b; }
void zero(int* data, int n) { memset(data, 0, sizeof(int) * n); }
void zero(ll* data, int n) { memset(data, 0, sizeof(ll) * n); }
void zero(char* data, int n) { memset(data, 0, sizeof(char) * n); }
char readc() { char var; fscanf(_fin, "%c", &var); return var; }
int readi() { int var; fscanf(_fin, "%d", &var); return var; }
ll readll() { ll var; fscanf(_fin, "%lld", &var); return var; }
void repread(int* data, int n) { repi(n) data[i] = readi(); }
void repread(ll* data, int n) { repi(n) data[i] = readll(); }
int reads(char* str, int maxsize)
{
	for (;;) { if (fgets(str, maxsize, _fin) == NULL) break; if (str[0] != '\n' && str[0] != '\r') break; }
	int slen = strlen(str); if (slen == 0) return 0;
	if (str[slen - 1] == '\n' || str[slen - 1] == '\r') str[--slen] = 0;
	return slen;
}

#define writec(var) fprintf(_fout,"%c",var)
#define writecsp(var) fprintf(_fout,"%c ",var)
#define writecln(var) fprintf(_fout,"%c\n",var)
#define writei(var) fprintf(_fout,"%d",var)
#define writeisp(var) fprintf(_fout,"%d ",var)
#define writellsp(var) fprintf(_fout,"%lld ",var)
#define writeiln(var) fprintf(_fout,"%d\n",var)
#define writellln(var) fprintf(_fout,"%lld\n",var)
#define writeulln(var) fprintf(_fout,"%llu\n",var)
#define writefln(var) fprintf(_fout,"%f\n",var)
#define writes(str) fprintf(_fout,"%s",str)
#define writesp() fprintf(_fout," ")
#define writeln() fprintf(_fout,"\n")
#define RUN_LOCAL(testfilename) {_fin = fopen(testfilename, "r"); if(_fin==NULL) _fin=stdin;}
#define swap(type,a, b) {type t=a;a=b;b=t;}
#define sort(data,n) std::sort(data,data+n)
//#define mod(a,b) b==0 ? 0 : a==0 ? 0 :(a>0 ? a%b : b + a % b) //what the hell is this...
ll gcd(ll a, ll b) { if (b == 0) return a; return gcd(b, a % b); }
ll lcm(ll a, ll b) { ll g = gcd(a, b); return (a / g) * b; }
void reverse(char* data, int n) { int k = n >> 1; repi(k) { char tmp = data[i]; data[i] = data[n - i - 1]; data[n - i - 1] = tmp; } }
void reverse(int* data, int n) { int k = n >> 1; repi(k) { int tmp = data[i]; data[i] = data[n - i - 1]; data[n - i - 1] = tmp; } }

#define _Vec(type, name) \
struct name { type* data; int size; int n; };\
void init(name* t, int size) { t->data = (type*)malloc(sizeof(type) * size); t->size = size; t->n = 0; }\
void resize(name* t) { int ns = t->size * 1.2f; t->data = (type*)realloc(t->data, sizeof(type) * ns); t->size = ns; }\
void add(name* t, type val) { if (t->n >= t->size) resize(t); int k = t->n; t->data[k] = val; t->n = k + 1; }\
void free(name* t) { free(t->data);}
_Vec(int, Veci)
_Vec(long long, Vecll)
_Vec(char*, Vecs)

#define _ispal(type, name) int name(type* a, type* b, int n){repi(n){if (a[i] != b[n - i - 1]){return 0;}}return 1;}
_ispal(int, ispali)
_ispal(char, ispalc)

#define _Pair(type, name) struct name{type x, y;}; \
int cmp_##name(const void* _a, const void* _b){name* a = (name*)_a;name* b = (name*)_b;if (a->x == b->x){if (a->y < b->y) return -1;else return 1;}if (a->x < b->x) return -1;else return 1;}\
void sort_##name(name* d, int n){qsort(d, n, sizeof(name), cmp_##name);}
	_Pair(int, Pairi)

	int cmp_Str(const void* _a, const void* _b) { char* a = *((char**)_a); char* b = *((char**)_b); return strcmp(a, b); }
void sort_Str(char** str, int n) { qsort(str, n, sizeof(char*), cmp_Str); }

ll expmod(ll x, ll n, ll m) {
	ll ans = 1;
	for (; n;) {
		if (n & 1) ans = (ans * x) % m;
		x = (x * x) % m;
		n >>= 1;
	}
	return ans;
}

ll combmod(ll n, ll k, ll m) {
	ll ret = 1;
	ll div = 1;
	for (ll i = 0; i < k; ++i) {
		ret = (ret * (n - i) % m) % m;
		div = (div * (i + 1)) % m;
	}
	div = expmod(div, m - 2, m) % m;
	return (ret * div) % m;
}
//------------------------------------------

//------------

int main()
{

	RUN_LOCAL("xxtestcase_05in.txt");
	static char s[ten5p1];
	static int count[26] = { 0 };
	int len = reads(s, ten5p1);
	repi(len) count[s[i]-'a']++;
	repi(26)
	{
		if (count[i] == 0)
		{
			printf("%c\n", i + 'a');
			return 0;
		}
	}
	writes("None\n");
	return 0;
}


Measured memory_bytes: 1752