// constants won't change. They're used here to set pin numbers, etc.
const int buttonPin = 15;
const int redLedPin = 2;
const int greenLedPin = 4;

// variables will change:
bool currButtonStatus = false; // to store the current button state
bool prevButtonStatus = false; // to store the previous button state
int count = 0;                 // to store the amount of button pressed

void setup() {
  // initialize the button pin as an input:
  pinMode(buttonPin, INPUT);
  // initialize the LED pins as output:
  pinMode(redLedPin, OUTPUT);
  pinMode(greenLedPin, OUTPUT);
}

void loop() {
  currButtonStatus = digitalRead(buttonPin);
  if (currButtonStatus == HIGH) {
    if (prevButtonStatus == false) {
      // increment count when button pressed and set previous button state to on
      count++;
      prevButtonStatus = !prevButtonStatus;
    }

    if (count % 2 == 0) {
      // turn on red LED
      digitalWrite(redLedPin, HIGH);
    }
    else {
      // turn on green LED
      digitalWrite(greenLedPin, HIGH);
    }
  } else {
    // turn off all LEDs and set previous button state to off
    digitalWrite(redLedPin, LOW);
    digitalWrite(greenLedPin, LOW);
    prevButtonStatus = false;
  }
  if (count >= 10) count %= 10;
}