/** || Bismillahir Rahmanir Rahim || **/

/**

FFFFFFFFFFFFFFFFFFFFFF                                                       iiii       DDDDDDDDDDDDD
F::::::::::::::::::::F                                                      i::::i      D::::::::::::DDD
F::::::::::::::::::::F                                                       iiii       D:::::::::::::::DD
FF::::::FFFFFFFFF::::F                                                                  DDD:::::DDDDD:::::D
  F:::::F       FFFFFF       aaaaaaaaaaaaa        rrrrr   rrrrrrrrr        iiiiiii        D:::::D    D:::::D
  F:::::F                    a::::::::::::a       r::::rrr:::::::::r       i:::::i        D:::::D     D:::::D
  F::::::FFFFFFFFFF          aaaaaaaaa:::::a      r:::::::::::::::::r       i::::i        D:::::D     D:::::D
  F:::::::::::::::F                   a::::a      rr::::::rrrrr::::::r      i::::i        D:::::D     D:::::D
  F:::::::::::::::F            aaaaaaa:::::a       r:::::r     r:::::r      i::::i        D:::::D     D:::::D
  F::::::FFFFFFFFFF          aa::::::::::::a       r:::::r     rrrrrrr      i::::i        D:::::D     D:::::D
  F:::::F                   a::::aaaa::::::a       r:::::r                  i::::i        D:::::D     D:::::D
  F:::::F                  a::::a    a:::::a       r:::::r                  i::::i        D:::::D    D:::::D
  F:::::F                 a::::a    a:::::a       r:::::r                 i::::::i     DDD:::::DDDDD:::::D
F::::::::FF                a:::::aaaa::::::a       r:::::r                 i::::::i     D:::::::::::::::DD
F::::::::FF                 a::::::::::aa:::a      r:::::r                 i::::::i     D::::::::::::DDD
FFFFFFFFFFF                  aaaaaaaaaa  aaaa      rrrrrrr                 iiiiiiii     DDDDDDDDDDDDD


	* Author   : Farid ul Islam Chy.
	* Batch    : 38th.
	* Semester : 7C(E).
	* Leading University, Sylhet.
	* Online Judge:
**/

#include <bits/stdc++.h>
using namespace std;

#define gc              getchar unlocked
#define gc              getchar
#define pc              putchar_unlocked
#define pc              putchar

#define pb    		    push_back
#define ForA(n)  	    for (ll i=0; i<n; i++)
#define ForA2(n)  	    for (ll j=0; j<n; j++)
#define ForA3(n)  	    for (ll k=0; k<n; k++)
#define ForN(n)  	    for (ll i=1; i<=n; i++)
#define ForN2(n)  	    for (ll j=1; j<=n; j++)
#define ForN3(n)  	    for (ll k=1; k<=n; k++)
#define SortA(n,x) 	    sort(n,n+x);
#define SortS(s)  	    sort(s.begin(), s.end());
#define SortRS(s)  	    sort(s.rbegin(), s.rend());
#define Search(s)       s.begin(), s.end()
#define SortSG(n)	    sort(n.begin(), n.end(), greater<int>());
#define Cin(a) 	        cin>>a;
#define GCin(a)	    	getline(cin,a);
#define Sz(a)		    a.size()
#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 scan2(a,b)      scan(a);scan(b);
#define scan3(a,b,c)    scan(a);scan(b),scan(c);
#define scan4(a,b,c,d)  scan(a);scan(b),scan(c),scan(d)
#define Unique(n)       sort(Search(n)),n.erase(unique(Search(n)),n.end())
#define PI 	            3.1415926535897932384626433832795l
#define Ignore	        cin.ignore();
#define Cout(n)		    cout<<n<<endl;
#define mem(n,i)        memset(n,i,sizeof n);
#define deci(n)         cout<<fixed<<setprecision(n);
#define square(n)       n*n
#define cube(n)         n*n*n
#define F               first
#define S               second
#define bug()		    cerr<<"*****<>*****\n";

typedef unsigned long long ull;
typedef long double ldb;
typedef long long ll;
typedef stringstream SS;
typedef vector<string> VS;
typedef vector<ll> VI;
typedef vector<char> VC;
typedef map<string, string> MSS;
typedef map <ll,string> MIS;
typedef map <char,ll> MCI;
typedef map <string,ll> MSI;
typedef map <ll,ll> MII;
typedef pair<int, int> pii;
map <ll, ll> fib;

const int dx4[] = {-1, 0, 1, 0};
const int dy4[] = { 0, 1, 0,-1};

const int dx8[] = {-1, 0, 1, 0, -1,-1, 1, 1};
const int dy8[] = { 0, 1, 0,-1, -1, 1,-1, 1};

const int dxhorse[] = {-2, -2, -1, -1, 1, 1, 2, 2};
const int dyhorse[] = {1,  -1, 2 , -2, 2,-2, 1,-1};

template <typename T> inline T ItoS(T n){SS a; a<<n;return a.str();}
template <typename T> inline T TrailingZeros(T  n){ll cnt = 0;for(ll i=5; n/i>=1; i*=5) cnt += n/i;return cnt;}
template <typename T> inline T StoI(T s){ll sum=0;for(ll i=0; i<Sz(s); i++){s[i]-='0';sum=sum*10+s[i];}return sum;}
template <typename T> inline T binary_decimal(T n){ll decimal=0, i=0, rem;while (n!=0){rem = n%10;n/=10;decimal += rem*pow(2,i);++i;}return decimal;}
template <typename T> inline T LCM(T a,T b){return (a*b)/GCD(a,b);}
template <typename T> inline T maxi(T x, T y) { if(x>y) return x; else return y;}
template <typename T> inline T maxi(T x, T y,T z) { return maxi(maxi(x,y),z);}
template <typename T> inline T mini(T x, T y) { if(x<y) return x; else return y;}
template <typename T> inline T mini(T x, T y,T z) { return mini(mini(x,y),z);}
template <typename T> inline T GCD(T a,T b)   {ll t;while(a){t = a;a = b%a;b = t;}return b;}
template <typename T> inline T Pow(T base,T exp){ll result = 1;while (exp){if(exp&1)result*=base;exp>>=1;base*=base;}return result;}
template <typename T> inline T Mulmod(T a, T b, T c){ull x = 0,y = a%c;while(b>0){if(b&1) x = (x+y)%c;y = (y<<1)%c;b >>= 1;}return x;}
template <typename T> inline T Pow(T base,T exp,T mod){ull x = 1, y = base;while(exp>0){if(exp&1) x = Mulmod(x,y,mod);y = Mulmod(y,y,mod);exp >>= 1;}return x;}
template <typename T> inline T modulo(T a,T b,T c){ull x = 1 , y = a%c ;while( b>0 ){if(b%2) x = Mulmod(x,y,c) ;y=Mulmod(y,y,c);b/=2;}return x%c ;}
template <typename T> inline T BigMod(T a, T b, T m){if(!b)return 1;ll x=BigMod(a, b/2, m);x=(x*x)%m;if(b%2)x=(x*a)%m;return x;}
template <typename T> inline T EGCD(T a,T b,T &x,T &y){if(a == 0) {x = 0;y = 1;return b;}T x1, y1;T d = EGCD(b % a, a, x1, y1);x = y1 - (b / a) * x1;y = x1;return d;}
template <typename T> inline T NumOfDivisor(T n){ ll cnt=0; int square_root = (int) sqrt(n) + 1; for (int i = 1; i < square_root; i++) { if (n%i == 0&&i*i!=n) cnt+=2; if (n%i == 0&&i*i==n) cnt++;} return cnt;}
template <typename T> string to_string(T const& value) { stringstream sstr; sstr << value; return sstr.str();}
template <class T1,class T2,class T3,class T4> int lcs( T1 X,T2 Y,T3 m,T4 n ){int L[m+1][n+1];int i, j;for (i=0; i<=m; i++){for (j=0; j<=n; j++){if (i == 0 || j == 0)L[i][j] = 0;else if (X[i-1] == Y[j-1])L[i][j] = L[i-1][j-1] + 1;else L[i][j] = max(L[i-1][j], L[i][j-1]);}}return L[m][n];}
template<class T>inline ll Fibonacci(T n){if(n == 0) return 0;if(n == 1 or n == 2) return fib[n] = 1;if(fib[n]) return fib[n];ll k = (n+1) >> 1;if(n&1){fib[n] = Fibonacci(k)*Fibonacci(k) + Fibonacci(k-1)*Fibonacci(k-1);}else fib[n] = ((Fibonacci(k-1)) << 1) + Fibonacci(k)*Fibonacci(k);return fib[n];}
template<class T, class T2>inline ll Fibonacci(T n, T2 M){if(n == 0) return 0;if(n == 1 or n == 2) return fib[n] = 1;if(fib[n]) return fib[n];ll k = (n+1) >> 1;if(n&1){fib[n] = (Fibonacci(k,M) * Fibonacci(k,M) + Fibonacci(k-1,M) * Fibonacci(k-1,M)) % M;}else fib[n] = (((Fibonacci(k-1,M)) << 1) + Fibonacci(k,M)*Fibonacci(k,M)) % M;return fib[n];}


/****************** Prime Generator **********************/

//const int N=1e7+10; int prime[20000010]; bool isprime[N]; int nprime;
//template <typename T> void Sieve(T a) {nprime = 0;memset(isprime,true,sizeof(isprime));isprime[1]=false;for(int i=2;i<N;i++){if(isprime[i]){prime[nprime++]=i;for(int j=2;i*j<N;j++)isprime[i*j]=false;}}}
//template <typename T> bool miller_rabin(T p, T itt) {if(p<2) return 0 ;if(p==2) return 1;if(p%2==0) return 0 ;ull s = p-1 ;while(s%2==0) s/=2;for(ll i=1;i<=itt;i++) {ull a = rand() % (p-1)+1 , temp = s ;ull mod = modulo(a,temp,(ull)p);while(mod!=1 and mod!=p-1 and temp!=p-1){mod = Mulmod(mod,mod,(ull)p);temp*=2;}if(temp%2==0 and mod!=p-1) return false ;}return true;}
//template <typename T> inline T PrimeFactors(T n){ll cnt=0,sum=1;for(int i=0; prime[i]*prime[i]<=n and i<nprime;i++){cnt=0;while(n%prime[i]==0){cnt++;n/=prime[i];}sum*=(cnt+1);}if(n>1)sum*=2;return sum;}

/********************** Faster I/O *********************/

template <typename T> inline T scan(T &n) { n=0; bool negative=0; char c=gc(); while(c<'0'||c>'9') {if(c=='-') negative=1; c=gc();} while(c>='0'&&c<='9'){n=n*10+c-48;c=gc();} if(negative) n=~(n-1);}
template <typename T> void print(T n){if(n<0){pc('-');n=-n;}if(!n){pc('0');pc('\n');return;}char buff[12];int len=0;while(n) buff[len++]=n%10+48,n/=10;for(int i=len-1;i>=0;i--) pc(buff[i]);pc('\n');}
template <typename T> void print(T a, T b){print(a);print(b);}
template <typename T> void print(T a, T b, T c){print(a);print(b);print(c);}
template <typename T> void Print(T n){if(n<0){pc('-');n=-n;}if(!n){pc('0');return;}char buff[12];int len=0;while(n) buff[len++]=n%10+48,n/=10;for(int i=len-1;i>=0;i--) pc(buff[i]);}
template <class T1,class T2> void Print(T1 e1,T2 e2){cout<<e1<<" "<<e2<<endl;}
template <class T1,class T2,class T3> void Print(T1 e1,T2 e2,T3 e3){cout<<e1<<" "<<e2<<" "<<e3<<"\n";}
template <class T1,class T2,class T3,class T4> void Print(T1 e1,T2 e2,T3 e3,T4 e4){cout<<e1<<" "<<e2<<" "<<e3<<" "<<e4<<"\n";}

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



int main(){

    #ifdef FariD
        freopen("in.txt","r",stdin);
        //freopen("out.txt","w",stdout);
    #endif // FariD

    ll n,m,e;

    string s;


    MCI(mp);
    Cin(s);

    ForA(Sz(s)){
        mp[s[i]]=1;
    }
    for(char i='a'; i<='z'; i++){
        if(mp[i]==0) return cout<<i<<endl, 0;
    }
    Cout("None")

    return 0;
}




Measured memory_bytes: 512