#include <Keypad.h>

const uint8_t ROWS = 4;
const uint8_t COLS = 4;
char keys[ROWS][COLS]={
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};

uint8_t colPins[COLS] = {5,4,3,2}; // pins connected to C1,C2,C3,C4:
uint8_t rowPins[ROWS] = {9,8,7,6}; // pins connected to R1,R2,R3,R4:

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

void setup(){
  Serial.begin(9600);
}

void loop() {
  char key = keypad.getKey();

  if(key != NO_KEY){
    Serial.print(key);
  }
}