function main(s) {
	s = s.split("\n")[0];
	var h = new Set();
	for (var i = 97; i <= 97+25; i++) {
		h.add(String.fromCharCode(i));
	}
	for (var i = 0; i < s.length; i++) {
		if (h.has(s[i])) {h.delete(s[i]);}
	}
	console.log(h.size === 0 ? "None" : Array.from(h).sort()[0]);
}
main(require("fs").readFileSync("/dev/stdin", "utf8"));

