import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;

void main() {
    bool[] alp = new bool[](26);
    string s = readln.chomp;

    foreach (ch ; s) {
        alp[ch - 'a'] = 1;
    }

    foreach (i ; 0 .. 26) {
        if (!alp[i]) {
            writeln(('a' + i).to!char);
            return;
        }
    }

    writeln("None");
}



void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}



void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}