Why I receive error NaN?

html

var countCorrectAnswers ;
var correctAnswers= [];
var wrongAnswers = [];


function print(message) {
  document.write(message);
}

function createList( list ) {
  var listHTML = '<ol>';
  for ( var i = 0; i < list.length; i += 1) {
    listHTML += '<li>' + list[i] + '</li>';
  }
  listHTML += '</ol>';
  return listHTML;
}


// two-dimensional array that holds quizzes and answers
var quizzes = [
  ['What is called the red planet?', 'mars'], 
  ['what is italian favorite sport?', 'soccer'], 
  ['What is the capital of Italy?', 'rome']
];



for (let i = 0; i < quizzes.length; i++) { 
  var answer = prompt(quizzes[i][0]);
  var answer = answer.toLowerCase();
  if (answer === quizzes[i][1]) {
    alert('correct answer!')
    countCorrectAnswers ++;
    correctAnswers.push(quizzes[i]);
  } else {
    alert('wrong answer!')
    wrongAnswers.push(quizzes[i]);
  }

}

print('<p> You guessed ' + countCorrectAnswers + ' answers right</p>')
print('<p> You got these quizzes right:</p>');
print(createList(correctAnswers));
print('<p> You got these quizzes wrong:</p>');
print(createList(wrongAnswers));