import std.stdio, std.string, std.conv, std.bigint, std.typecons, std.algorithm, std.array, std.math, std.range, std.functional;
 
void main() {
    auto S = sort(readln.chomp.map!(c => cast(int)(c - 'a')).array).uniq;
    auto res = iota('z' - 'a' + 1).filter!(c => !S.canFind(c));
    writeln(res.empty ? "None" : (cast(char)(res.front + 'a')).to!string);
}