#include <Wire.h>
#define SLAVE_ADDRESS   0x20
byte  reTxData;
volatile bool flag = false;

void setup()
{
  Wire.setClock(1000000);
  Serial.begin(9600);
  Wire.onReceive(receiveEvent);
  Wire.onRequest(sendEvent);
  Wire.begin(SLAVE_ADDRESS);
}

void loop()
{
  if (flag == true)
  {
    Serial.print("Data received from Master: ");
    Serial.println(reTxData);
    flag = false;
  }
}

void receiveEvent(int numBytes)
{
  reTxData = Wire.read();
  flag = true;
}

void sendEvent()
{
  Wire.write(reTxData);
}