import std.stdio, std.string, std.uni, std.algorithm, std.array, std.conv;

void main() {
    char[] s = readln.strip.dup;
    char[] lowers = s.filter!isLower.map!(to!char).array;
    char[] alphabet = "abcdefghijklmnopqrstuvwxyz".dup;

    auto filtered = alphabet.filter!(c => !lowers.canFind(c)).array.sort;

    if (filtered.length > 0) {
        writeln(filtered[0]);
    }
    else {
        writeln("None");
    }

}
