'use strict';

const main = (input) => {
  const [N] = input.trim();
  let num = Number(N);
  let count = 0;
  num -= 3;
  while (true) {
    if (num % 2 === 0) {
      count++;
      num -= 2;
      if (num === 0) {
        break;
      } else {
        break;
      }
    }
  }
  console.log(count);
};
main(require('fs').readFileSync('/dev/stdin', 'utf8'));