#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) x.begin(), x.end()
using namespace std;
typedef long long ll;
typedef pair<int, int> pair;
const int inf = 1000000000; //10^9
const ll inff = 1000000000000000000; //10^18

int main(){
    string s;
    cin >> s;
    int n = s.size();
    bool a[26];
    rep(i, 26) a[i] = false;

    rep(i, n) {
        a[s[i] - 'a'] = true;
    }

    rep(i, 26) {
        if(!a[i]) {
            cout << (char)(i + 'a') << endl;
            return 0;
        }
    }

    cout << "None" << endl;

    return 0;
}
#include <iostream>
#include <string>
#include <map>
#include <string.h>
using namespace std;
int main(){
  map<char,int> ch;//文字の数字辞書登録
  ch['I']=1;
  ch['V']=5;
  ch['X']=10;
  ch['L']=50;
  ch['C']=100;
  ch['D']=500;
  ch['M']=1000;
  
  int ans=0;
  int len=0;
  char s[1111];
  while(cin >> s){
    len=strlen(s);
    for(int i=0;i<len;i++){
      if(ch[s[i]]<ch[s[i+1]]){//もしも引き算をすべき場合だったら
	ans-=ch[s[i]];
      }else{
	ans+=ch[s[i]];
      }
    }
    cout << ans << endl;
    ans=0;
  }
}