const byte ROWS = 4;  //four rows
const byte COLS = 4;  //four columns
//define the characters on the buttons of the keypad
char keys[ROWS][COLS] = {  
  {'1','2','3','D'},
  {'4','5','6','x'},
  {'7','8','9','y'},
  {'C','0','.','='}
};

byte rowPins[ROWS] = {6, 7, 8, 9};  //connect to the row pinouts of the keypad
byte colPins[COLS] = {5, 4, 3, 2};  //connect to the column pinouts of the keypad
// Create the Keypad object
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );