#include <iostream>
#include <string>
#include <algorithm>
#include <cmath>
#include <stack>
#include <vector>
#include <deque>
#include <queue>
#define _CRT_SECURE_NO_WARNINGS
#define JFT20 ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ss string
#define ll long long
#define big INT_MAX
#define small INT_MIN
#define bigg LONG_MAX
#define smalll LONG_MIN
#define all(x) (x.begin(),x.end())
#define sz(x) x.size()
#define nn int nn; cin>>nn; int n=nn;
using namespace std;
int main() {
JFT20
ss str;
cin >> str;
char test = 'a';
for (int i = 0;i < sz(str);i++) {
str[i] = tolower(str[i]);
if (str[i] != test && str[i] != test - 1) {
cout << test;
break;
}
else {
test++;
continue;
}
}
return 0;
}
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