#include <bits/stdc++.h>
using namespace std;
  #define IOS ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
  #define int long long
  #define endl "\n"
  #define all(v) v.begin(),v.end()
  #define double long double
  #define trace1(x)                cerr<<#x<<": "<<x<<endl
  #define trace2(x, y)             cerr<<#x<<": "<<x<<" | "<<#y<<": "<<y<<endl
  #define trace3(x, y, z)          cerr<<#x<<":" <<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl
  #define trace4(a, b, c, d)       cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<endl
  #define trace5(a, b, c, d, e)    cerr<<#a<<": "<<a<<" | "<<#b<< ": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<< ": "<<e<<endl
  #define trace6(a, b, c, d, e, f) cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<< ": "<<e<<" | "<<#f<<": "<<f<<endl
  #define cases int testcases;cin>>testcases; while(testcases--)
  const int N = 200005;


  int dp[101][101];

int32_t main()
{


    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif
    IOS


    string s;
    cin >> s;

    set<char> ss(s.begin(),s.end());

    for(int i = 0; i < 26; i++)
    {
      char ch = 'a' + i;
      if(!ss.count(ch))
      {
        cout << ch;
        return 0;
      }
    }

    cout << "None";
    return 0;   
}

#include <cstdio>

char S[101010];

int main()
{
	scanf("%s", S);

	bool app[26];
	for (int i = 0; i < 26; ++i) {
		app[i] = false;
	}
	for (int i = 0; S[i] != '\0'; ++i) {
		app[S[i] - 'a'] = true;
	}
	for (int i = 0; i < 26; ++i) {
		if (!app[i]) {
			printf("%c\n", (char)(i + 'a'));
			return 0;
		}
	}
	puts("None");
	return 0;
}
