main( argv_ ) {
	if ( size( argv_ ) < 2 ) {
		throw Exception( "usage: digital-root {NUM}" );
	}
	n = argv_[1];
	if ( ( size( n ) == 0 ) || ( n.find_other_than( "0123456789" ) >= 0 ) ) {
		throw Exception( "{} is not a number".format( n ) );
	}
	shift = integer( '0' ) + 1;
	acc = 0;
	for ( d : n ) {
		acc = 1 + ( acc + integer( d ) - shift ) % 9;
	}
	print( "{}\n".format( acc ) );
	return ( 0 );
}