how can i keep a value before i updated?

See more:

C#

how can keep a value which it was recorded in database in variable before i updated it? i want the old value for an operation with new value,
example:
NewValue - OldValue = VariableValue
230 - 200 = 30,
by the way the OldValue is NewValue before i updated it.
can u give me an idea about it, if it's possible.

private void ChargerKiloVoitures()
{
			
try {
	_bd.Command.CommandText = "SELECT Kilometrage FROM Voitures " +
	"WHERE NumeroMatricule = \"" + CBNumMatricule.Text + "\"";
	Debug.WriteLine( Environment.NewLine );
	Debug.WriteLine( _bd.Command.CommandText );
	_bd.Reader = _bd.Command.ExecuteReader();
if ( _bd.Reader.Read());
	this.analogCounter2.Number = int.Parse(_bd.Reader["Kilometrage"].ToString());
	this.analogCounter1.Number = this.analogCounter2.Number + VidangeConstant;
					
					
   //i have problem here exactly 
	int OldKilometrage = int.Parse(_bd.Reader["Kilometrage"].ToString());
	int NewKilometrage = int.Parse(_bd.Reader["Kilometrage"].ToString());
	KiloMetrag.Value = NewKilometrage-OldKilometrage;
					
}//if
	_bd.Reader.Close();
 
}//try
	catch (Exception ex) {
		Debug.WriteLine( Environment.NewLine );
		Debug.WriteLine( ex.Message );
	MessageBox.Show("Une erreur a été détectée : " + Environment.NewLine + ex.Message, "Erreur");
	}//catch

}