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

using ll = long long;
//print(fixed << setprecision(10));
#define rep(i,n) for(ll i = 0; i < (n); i++)
#define repk(i,n,k) for(ll i = k; i < (n); i++)
#define all(n) n.begin(), n.end()
#define print(n) cout << n << endl
double PI = acos(-1.0);
int INF = 1e9;
ll LINF = 1e18;
ll MOD = 1e9+7;


int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    
    ll n;
    cin >> n;
    string s="";
    while(n){
        n--;
        s+=('a'+(n+25/26)%26);
        n/=26;
    }
    reverse(all(s));
    print(s);
    return 0;
}
#include <bits/stdc++.h>
using namespace std;

int main(void) {
  string s; cin >> s;
  string c = "abcdefghijklmnopqrstuvwxyz";
  bool find_flag = false;
  
  for (int i=0; i<26; i++) {
    bool target = false;
    for (int j=0; j<s.length(); j++) {
      if (c[i] == s[j]) target = true;
    }
    if (!target) {
      cout << c[i] << endl;
      find_flag = true;
      break;
    }
  }
  
  if (!find_flag) cout << "None" << endl;
  return 0;
}