Module a

ABC 404 A - Not Found

refs: https://atcoder.jp/contests/abc404/tasks/abc404_a

use proconio::input;

fn main() {
    input! {
      s: String,
    }

    let alphabets = b'a'..=b'z';

    for alphabet in alphabets {
        if s.contains(alphabet as char) {
            continue;
        }

        println!("{}", alphabet as char);
        return;
    }
}