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 set = setDifference(alphabet, lowers);

    if (!set.empty) {
	writeln(set.front);
    }
    else {
	writeln("None");
    }
}
