How to make a bidirectional visitor counter

#include <LiquidCrystal.h>
#define trigPin 18
#define echoPin 19

LiquidCrystal lcd(2,3,4,5,6,7);
int counter = 0;
int currentState1 = 0;
int previousState1 = 0;
int currentState2 = 0;
int previousState2 = 0;
int inside = 0;
int outside = 0;

void setup()
{
 pinMode(trigPin, OUTPUT);
 pinMode(echoPin, INPUT);
 
 lcd.begin(16,2);
 lcd.setCursor(0,0);
 lcd.print(" Visitor");
 lcd.setCursor(0,1);
 lcd.print(" Counter");
 delay(8000);
 lcd.clear();
 lcd.print(" Shashwat__Raj");
 delay(8000);
 lcd.clear();
}

void loop()
{
lcd.begin(16,2);   // iInit the LCD for 16 chars 2 lines
lcd.setCursor(0, 0);
lcd.print("IN: ");
lcd.setCursor(7, 0);
lcd.print("OUT: ");
lcd.setCursor(0, 1);
lcd.print("Total Inside: ");
long duration, distance;
digitalWrite(trigPin,LOW);
delayMicroseconds(2);
digitalWrite(trigPin,HIGH);
delayMicroseconds(10);
digitalWrite(trigPin,LOW);
duration = pulseIn(echoPin, HIGH);
distance = (duration/2) / 29.1;
if (distance <= 9){
currentState1 = 1;
}
else {
currentState1 = 0;
}
delay(100);
if(currentState1 != previousState1){
if(currentState1 == 1){
counter = counter + 1;}
lcd.setCursor(14, 1);
lcd.print(counter);
inside = inside +1;}
lcd.setCursor(4, 0);
lcd.print(inside);
if (distance > 9 && distance <= 18){
currentState2 = 1;
}
else {
currentState2 = 0;
}
delay(100);
if(currentState2 != previousState2){
if(currentState2 == 1){
counter = counter - 1;}
lcd.setCursor(14, 1);
lcd.print(counter);
outside = outside +1;}
lcd.setCursor(12, 0);
lcd.print(outside);
lcd.setCursor(14, 1);
lcd.print(counter);
if (counter > 9 || counter < 0){
lcd.setCursor(14, 1);
lcd.print(counter);
delay(100);
lcd.clear();
 }
}