function calcular_distancia()
{
   	error = false;
   	if (form_calculadora.select_destino_inicial.value == 0)
   	{
   		alert('The inicial destiny is requiered.');
	  	form_calculadora.select_destino_inicial.focus();
		error = true;
   	}
   	if ((!error)&&(form_calculadora.select_destino_final.value == 0))
   	{
   		alert('The final destiny is requiered.');
	  	form_calculadora.select_destino_final.focus();
		error = true;
	}
   	if ((!error)&&(form_calculadora.select_destino_inicial.value == form_calculadora.select_destino_final.value))
   	{
		alert('The inicial destiny must be different at the final destiny.');
		form_calculadora.select_destino_final.focus();
		error = true;
	}
	if (!error)
    {
	   form_calculadora.submit();
    }
}