// Try AtCoder
/// author: Leonardone @ NEETSDKASU
import std.stdio     : readln, writeln;
import std.string    : chomp;
import std.array     : split;
import std.conv      : to;

auto getVals(T)() { return readln.chomp.split.to!(T[]); }

void main() {
    bool[char] table;
    foreach (ch; readln.chomp) {
        table[ch] = true;
    }
    for (auto ch = 'a'; ch <= 'z'; ch++) {
        if (ch !in table) {
            writeln(ch);
            return;
        }
    }
    writeln("None");
}