Problem with RC522 RFID Reader/Writter

I try the code at the RC522 RFID Library about the read and write tag function. When I try to modified the code to sent a integer (int num = 500; ) in the Tag 2 for calculation, the serial print show out a the value is not 500. I try many times and it still wrong. What's wrong with my coding?

#include <SPI.h>//include the SPI bus library
#include <MFRC522.h>//include the RFID reader library

#define SS_PIN 10  //slave select pin
#define RST_PIN 5  //reset pin
MFRC522 mfrc522(SS_PIN, RST_PIN);        // instatiate a MFRC522 reader object.
MFRC522::MIFARE_Key key;//create a MIFARE_Key struct named 'key', which will hold the card information

void setup() {
        Serial.begin(9600);        // Initialize serial communications with the PC
        SPI.begin();               // Init SPI bus
        mfrc522.PCD_Init();        // Init MFRC522 card (in case you wonder what PCD means: proximity coupling device)
        Serial.println("Scan a MIFARE Classic card");
        // Prepare the security key for the read and write functions - all six key bytes are set to 0xFF at chip delivery from the factory.
        // Since the cards in the kit are new and the keys were never defined, they are 0xFF
        // if we had a card that was programmed by someone else, we would need to know the key to be able to access it. This key would then need to be stored in 'key' instead.
        for (byte i = 0; i < 6; i++) {
                key.keyByte[i] = 0xFF;//keyByte is defined in the "MIFARE_Key" 'struct' definition in the .h file of the library


int block=2;//this is the block number we will write into and then read. Do not write into 'sector trailer' block, since this can make the block unusable.
int num = 500;

byte blockcontent[16] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};//an array with 16 bytes to be written into one of the 64 card blocks is defined
//byte blockcontent[16] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};//all zeros. This can be used to delete a block.
byte readbackblock[18];//This array is used for reading out a block. The MIFARE_Read method requires a buffer that is at least 18 bytes to hold the 16 bytes of a block.

void loop()

        /*****************************************establishing contact with a tag/card**********************************************************************/
  	// Look for new cards (in case you wonder what PICC means: proximity integrated circuit card)
	if ( ! mfrc522.PICC_IsNewCardPresent()) {//if PICC_IsNewCardPresent returns 1, a new card has been found and we continue
		return;//if it did not find a new card is returns a '0' and we return to the start of the loop

	// Select one of the cards
	if ( ! mfrc522.PICC_ReadCardSerial()) {//if PICC_ReadCardSerial returns 1, the "uid" struct (see MFRC522.h lines 238-45)) contains the ID of the read card.
		return;//if it returns a '0' something went wrong and we return to the start of the loop

        // Among other things, the PICC_ReadCardSerial() method reads the UID and the SAK (Select acknowledge) into the mfrc522.uid struct, which is also instantiated
        // during this process.
        // The UID is needed during the authentication process
        	//The Uid struct:
	        //typedef struct {
		//byte		size;			// Number of bytes in the UID. 4, 7 or 10.
		//byte		uidByte[10];            //the user ID in 10 bytes.
		//byte		sak;			// The SAK (Select acknowledge) byte returned from the PICC after successful selection.
	        //} Uid;
         Serial.println("card selected");
         /*****************************************writing and reading a block on the card**********************************************************************/
         readBlock(block, readbackblock);
         int temp = int(readbackblock[18] + num );
         blockcontent[16] += temp;
         writeBlock(block, blockcontent);
         readBlock(block, readbackblock);//read the block back
         int show = int(readbackblock);
         Serial.print("read block: ");