Keypad Calculator using LCD

#include <Keypad.h>
#include <LiquidCrystal.h>
int Contrast = 120;
LiquidCrystal lcd(A1, A2, 13, 12, 11, 10);

const byte ROWS = 4; 
const byte COLS = 4; 
int expr[2];
int i=0;
char op;

void resetCalc()
{
  	Serial.println();
  	i = 0;
  	lcd.clear();
  	lcd.setCursor(0,0);
}
char hexaKeys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

byte rowPins[ROWS] = {9, 8, 7, 6}; 
byte colPins[COLS] = {5, 4, 3, 2}; 

Keypad customKeypad = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS); 
void setup(){
  Serial.begin(9600);
  analogWrite(A0, Contrast);
  lcd.begin(16, 2);
  lcd.noCursor();
  lcd.setCursor(0,0);
  Serial.println("Welcome to keypad Calculator");
  Serial.println("A = +, B = -, C = *, D = /");
  Serial.println("Press # to calculate");
  Serial.println("Press * to reset");
  Serial.println("NOTE: It works only on single digit numbers\n\n");
}
  
void loop(){
  char key = customKeypad.getKey();
  if(key)
  {
    if(i == 0)
    {
          expr[0] = key-48;
          Serial.print(expr[0]);
      	  lcd.print(expr[0]);
          i++;
    }
    else if(i == 1)
    {
          op = key;
      	  i++;
     	  if(op == 'A')
          {
          	Serial.print(" + ");
            lcd.print(" + ");
          }
      	  else if(op == 'B')
          {
          	Serial.print(" - ");
            lcd.print(" - ");
          }
          else if(op == 'C')
          {
          	Serial.print(" * ");
            lcd.print(" * ");
          }
          else if(op == 'D')
          {
          	Serial.print(" / ");
            lcd.print(" / ");
          }
          else
          {
            Serial.println("Syntax error");
            lcd.print(" Syntax error ");
            resetCalc();
          }
         
    }
    else if(i == 2)
    {
          expr[1] = key-48;
          Serial.print(expr[1]);
      	  lcd.print(expr[1]);
      	  Serial.print(" = ");
      	  lcd.print(" = ");
          if(op == 'A')
          {
            Serial.println(expr[0] + expr[1]);
            lcd.print(expr[0] + expr[1]);
          }
          else if(op == 'B')
          {
            Serial.println(expr[0] - expr[1]);
            lcd.print(expr[0] - expr[1]);
          }
          else if(op == 'C')
          {
            Serial.println(expr[0] * expr[1]);
            lcd.print(expr[0] * expr[1]);
          }
          else if(op == 'D')
          {
            Serial.println(expr[0] / expr[1]);
            lcd.print(expr[0] / expr[1]);
          }

          i++;
    }
    else if(key == '*' || i == 3)
    {
      resetCalc();
    }
  }
}