Problem:
Score : 200 points

Problem 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.

Constraints

1 \leq |S| \leq 10^5 (|S| is the length of string S.)

S consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

S

Output

Print the lexicographically smallest lowercase English letter that does not occur in S.
If every lowercase English letter occurs in S, print None instead.

Sample Input 1

atcoderregularcontest

Sample Output 1

b

The string atcoderregularcontest contains a, but does not contain b.

Sample Input 2

abcdefghijklmnopqrstuvwxyz

Sample Output 2

None

This string contains every lowercase English letter.

Sample Input 3

fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg

Sample Output 3

d

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <vector>
using namespace std;
typedef long long int ll;
typedef long double ld;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define Sort(Array) sort(Array.begin(), Array.end())
#define Reverse(a) reverse(a.begin(), a.end())
#define out(ans) cout << ans << endl;
const int MOD = 1000000007;
const int INF = 2147483647;
const ld PI = 3.14159265358979323846;

//------------↓------------- M -------------- E ---------------- M --------------- O ---------------↓--------------//
// コンパイル 
// g++ -std=c++1z
//
// -------型変換--------
// int を string に変換
// string s = to_string(n);
// string を int に変換
// int n = stoi(s);
//
// -------二分探索---------
// k以上の値が最初に現れる時のイテレータ
// lower_bound(data.begin(), data.end(), k)
// kより大きい値が最初の現れる時のイテレータ O(logN)
// upper_bound(data.begin(), data.end(), k)
// kがdataに存在するかをtrue or falseで返す O(logN)
// binary_search(data.begin(), data.end(), k)
// 
//
//
//
//
//
// 
//------------↑------------- M -------------- E ---------------- M --------------- O ---------------↑--------------//







int main(void) {

    string s; cin >> s;
    int l = 'z' - 'a' + 1; 
    bool flag[l];
    rep(i,l) flag[i] = false;
    rep(i,s.size()) {
        int num = s[i] - 'a';
        flag[num] = true;
    }
    bool flag_2 = true;
    rep(i,l) {
        if (!flag[i]) {
            char c = i + 'a';
            out(c);
            flag_2 = false;
            break;
        }
    }
    if (flag_2) cout << "None" << endl;

    
    
    return 0;
}