#include<iostream>
#include<iomanip>
#include<cmath>
#include<string>
#include<cstring>
#include<vector>
#include<list>
#include<algorithm>
#include<map>
#include<set>
#include<queue>
#include<stack>
using namespace std;
typedef long long ll;
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
#define pqueue priority_queue
const int inf=1e9+7;
const ll mod=1e9+7;
const ll mod1=998244353;
const ll big=1e18;
const double PI=2*asin(1);

int main() {
  string S;
  cin>>S;
  map<char, int> amap;
  for(int i=0;i<S.size();++i) {
    amap[S[i]]++;
  }
  char ans = 'A';
  for(char a='a';a<='z';++a) {
    if(amap[a]==0) {
      ans = a;
      break;
    }
  }
  if(ans=='A') cout<<"None"<<endl;
  else cout<<ans<<endl;
}


#include<iostream>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i,n) for(int i=0;i<n;i++)
using P = pair<int, int>;

/* 大文字を小文字に変換 */
char tolower(char c) {return (c + 0x20);}
/* 小文字を大文字に変換 */
char toupr(char c) {return (c - 0x20);}

// if('A'<=s[i] && s[i]<='Z') s[i] += 'a'-'A';

/*
string s = "abcdefg"
s.substr(4) "efg"
s.substr(0,3) "abc"
s.substr(2,4) "cdef"

// イテレータ要素のインデックス
distance(A.begin(), itr);
*/

int main()
{
    int a,b,c; cin >> a >> b >> c;
    int cnt = 0;
    if(a == b) cnt++;
    if(a == c) cnt++;
    if(b == c) cnt++;
    if(cnt == 1) cout << "Yes" << endl;
    else cout << "No" << endl;
}