#include <Wire.h>

#define SLAVE_ADDRESS 0x08

const int buttons[6] = {D0, D1, D7, D8, D9, D10};

void setup() {
  Wire.begin(); // Master

  for (int i = 0; i < 6; i++) {
    pinMode(buttons[i], INPUT);
  }
}

void loop() {

  for (int i = 0; i < 6; i++) {

    if (digitalRead(buttons[i]) == HIGH) {

      Wire.beginTransmission(SLAVE_ADDRESS);
      Wire.write(i + 1);
      Wire.endTransmission();

      delay(300);
    }
  }
}