#include "Wire.h"

const byte I2C_SLAVE_ADDR = 0x20;

long data = 100;
long response = 0;

void setup()
{
   Serial.begin(115200);
   Wire.begin();
}

void loop()
{
   sendToSlave();
   requestToSlave();
   delay(2000);
}

void sendToSlave()
{
   Wire.beginTransmission(I2C_SLAVE_ADDR);
   Wire.write((byte*)&data, sizeof(data));
   Wire.endTransmission();
}

void requestToSlave()
{
   response = 0;
   Wire.requestFrom(I2C_SLAVE_ADDR, sizeof(response));

   uint8_t index = 0;
   byte* pointer = (byte*)&response;
   while (Wire.available())
   {
      *(pointer + index) = (byte)Wire.read();
      index++;
   }

   Serial.println(response);
}