OIG XVI - zag

// https://szkopul.edu.pl/problemset/problem/ti5j6b9KtyB8v1g1DUD6RHX7/site/?key=statement
// https://oij.edu.pl/oij16/etap3/zadania/zag/zagzad.pdf

// tests:
// https://oij.edu.pl/oij16/etap3/zadania/zag/zag.zip

#include <array>
#include <iostream>

// bitset would be better
std::array<bool, 1000> arr;

int main() {
    std::ios_base::sync_with_stdio(0);
    std::cin.tie(0);
    std::cout.tie(0);

    std::string s;
    std::cin >> s;

    for (const char& c : s) {
        arr[c - 'A'] = true;
    }

    for (int i = 0; i < 1000; i++) {
        if (!arr[i]) {
            std::cout << (char)(i + 'A') << '\n';
            return 0;
        }
    }

    return 0;
}