output_description: Print the lexicographically smallest lowercase English letter that does not
  occur in S. If every lowercase English letter occurs in S, print `None`
  instead.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      S
attempt: #include <bits/stdc++.h>
  using namespace std;
  #define rep(i,N) for(int i=0,i##_max=(N);i<i##_max;++i)
  #define repp(i,l,r) for(int i=(l),i##_max=(r);i<i##_max;++i)
  #define per(i,N) for(int i=(N)-1;i>=0;--i)
  #define perr(i,l,r) for(int i=r-1,i##_min(l);i>=i##_min;--i)
  #define all(arr) (arr).begin(), (arr).end()
  #define SP << " " <<
  #define SPF << " "
  #define SPEEDUP cin.tie(0);ios::sync_with_stdio(false);
  #define MAX_I INT_MAX //1e9
  #define MIN_I INT_MIN //-1e9
  #define MAX_UI UINT_MAX //1e9
  #define MAX_LL LLONG_MAX //1e18
  #define MIN_LL LLONG_MIN //-1e18
  #define MAX_ULL ULLONG_MAX //1e19
    typedef long long ll;
    typedef pair<int,int> PII;
    typedef pair<char,char> PCC;
    typedef pair<ll,ll> PLL;
    typedef pair<char,int> PCI;
    typedef pair<int,char> PIC;
    typedef pair<ll,int> PLI;
    typedef pair<int,ll> PIL; 
    typedef pair<ll,char> PLC; 
    typedef pair<char,ll> PCL; 
  
  inline void YesNo(bool b){ cout << (b?"Yes" : "No") << endl;}
  inline void YESNO(bool b){ cout << (b?"YES" : "NO") << endl;}
  inline void Yay(bool b){ cout << (b?"Yay!" : ":(") << endl;}
  
  int main(void){
    SPEEDUP
    cout << setprecision(15);
    string s;cin >> s;
    sort(all(s));
    s.erase(unique(all(s)),s.end());
    bool isNone = true;
    rep(i,26){
      char out = 'a'+i;
      if(i==s.length()){
        cout << out << endl;
        isNone = false;
        break;
      }
      if(s[i] != 'a'+i){
        cout << out << endl;
        isNone = false;
        break;
      }
    }
    if(isNone) cout << "None" << endl;
    return 0;
  }
  
problem_description: Statement
  
  You are given a string S consisting of lowercase English letters. Find the
  lexicographically (alphabetically) smallest lowercase English letter that does
  not occur in S. If every lowercase English letter occurs in S, print `None`
  instead.
samples: [{"input": "atcoderregularcontest", "output": "b\n    \n\nThe string `atcoderregularcontest` contains `a`, but does not contain `b`.\n\n* * *"}, {"input": "abcdefghijklmnopqrstuvwxyz", "output": "None\n    \n\nThis string contains every lowercase English letter.\n\n* * *"}, {"input": "fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg", "output": "d"}]