#include<iostream>
#include<string>
#include<algorithm>	
#include<cmath>
using namespace std;
int main() {
	long long a = 0, b = 0, c = 0, d = 0, e = 0, f = 0, g = 0, h = 0, i = 0, j = 0, k = 0, l = 0, m = 0;
	long long n = 0, o = 0, p = 0, q = 0, r = 0, s = 0, t = 0, u = 0, v = 0, w = 0, x = 0, y = 0, z = 0;
	string ss; cin >> ss;
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'a') { a++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'b') { b++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'c') { c++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'd') { d++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'e') { e++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'f') { f++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'g') { g++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'h') { h++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'i') { i++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'j') { j++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'k') { k++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'l') { l++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'm') { m++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'n') { n++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'o') { o++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'p') { p++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'q') { q++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'r') { r++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 's') { s++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 't') { t++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'u') { u++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'v') { v++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'w') { w++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'x') { x++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'y') { y++; } }
	for (int hi = 0; hi < ss.size(); hi++) { if (ss[hi] == 'z') { z++; } }
	if (a == 0) { cout << 'a' << endl; cin >> n; return 0; }
	if (b == 0) { cout << 'b' << endl; cin >> n; return 0; }
	if (c == 0) { cout << 'c' << endl; cin >> n; return 0; }
	if (d == 0) { cout << 'd' << endl; cin >> n; return 0; }
	if (e == 0) { cout << 'e' << endl; cin >> n; return 0; }
	if (f == 0) { cout << 'f' << endl; cin >> n; return 0; }
	if (g == 0) { cout << 'g' << endl; cin >> n; return 0; }
	if (h == 0) { cout << 'h' << endl; cin >> n; return 0; }
	if (i == 0) { cout << 'i' << endl; cin >> n; return 0; }
	if (j == 0) { cout << 'j' << endl; cin >> n; return 0; }
	if (k == 0) { cout << 'k' << endl; cin >> n; return 0; }
	if (l == 0) { cout << 'l' << endl; cin >> n; return 0; }
	if (m == 0) { cout << 'm' << endl; cin >> n; return 0; }
	if (n == 0) { cout << 'n' << endl; cin >> n; return 0; }
	if (o == 0) { cout << 'o' << endl; cin >> n; return 0; }
	if (p == 0) { cout << 'p' << endl; cin >> n; return 0; }
	if (q == 0) { cout << 'q' << endl; cin >> n; return 0; }
	if (r == 0) { cout << 'r' << endl; cin >> n; return 0; }
	if (s == 0) { cout << 's' << endl; cin >> n; return 0; }
	if (t == 0) { cout << 't' << endl; cin >> n; return 0; }
	if (u == 0) { cout << 'u' << endl; cin >> n; return 0; }
	if (v == 0) { cout << 'v' << endl; cin >> n; return 0; }
	if (w == 0) { cout << 'w' << endl; cin >> n; return 0; }
	if (x == 0) { cout << 'x' << endl; cin >> n; return 0; }
	if (y == 0) { cout << 'y' << endl; cin >> n; return 0; }
	if (z == 0) { cout << 'z' << endl; cin >> n; return 0; }
	cout << "None" << endl;
	cin >> n;
}

Measured memory_bytes: 512