#include <Keypad.h>

const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  {'1','2','3','+'},
  {'4','5','6','-'},
  {'7','8','9','*'},
  {'*','0','=','/'}
};

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

Keypad keypad = Keypad(makeKeymap(keys),rowPins,colPins,ROWS, COLS);

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial.println("Enter Key:");
}

void loop() {
  // put your main code here, to run repeatedly:
  char value = keypad.getKey();

  if(value != NO_KEY) {
    Serial.println(value);
  }

}