unittest
{
	assert( [ "atcoderregularcontest" ].parse.expand.solve == "b" );
	assert( [ "abcdefghijklmnopqrstuvwxyz" ].parse.expand.solve == "None" );
	assert( [ "fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg" ].parse.expand.solve == "d" );

	assert( [ "acb" ].parse.expand.solve == "d" );		// 追加ケース
}



import std.algorithm;
import std.conv;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

void main()
{
	stdin.byLineCopy.parse.expand.solve.writeln;
}

auto parse( Range )( Range input )
if( isInputRange!Range && is( ElementType!Range == string ) )
{
	auto s = input.front.strip.to!( dchar[] );
	return tuple( s );
}

auto solve( dchar[] s )
{
	auto t = s.sort().uniq;
	foreach( c; 'a' .. 'z' )
	{
		if( t.empty || t.front != c ) return c.to!( string );
		t.popFront;
	}
	return "None";
}
