import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

void readV(T...)(ref T t){auto r=readln.splitter;foreach(ref v;t){v=r.front.to!(typeof(v));r.popFront;}}

void main()
{
  string s; readV(s);

  auto b = new bool[](26);
  foreach (c; s) b[c-'a'] = true;

  foreach (i; 0..26)
    if (!b[i]) {
      writeln(dchar(i+'a'));
      return;
    }

  writeln("None");
}
