import std.algorithm;
import std.ascii;
import std.array;
import std.conv;
import std.math;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

int readint() {
    return readln.chomp.to!int;
}

int[] readints() {
    return readln.split.map!(to!int).array;
}

void main() {
    auto s = readln.chomp;

    bool found = false;
    foreach (c; lowercase) {
        auto p = s.indexOf(c);
        if (p == -1) {
            writeln(c);
            found = true;
            break;
        }
    }

    if (!found)
        writeln("None");
}
