Problem:
Score : 500 points

Problem Statement

For an integer n not less than 0, let us define f(n) as follows:

f(n) = 1 (if n < 2)

f(n) = n f(n-2) (if n \geq 2)

Given is an integer N. Find the number of trailing zeros in the decimal notation of f(N).

Constraints

0 \leq N \leq 10^{18}

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of trailing zeros in the decimal notation of f(N).

Sample Input 1

12

Sample Output 1

1

f(12) = 12 × 10 × 8 × 6 × 4 × 2 = 46080, which has one trailing zero.

Sample Input 2

5

Sample Output 2

0

f(5) = 5 × 3 × 1 = 15, which has no trailing zeros.

Sample Input 3

1000000000000000000

Sample Output 3

124999999999999995

Solution in JavaScript (node.js v5.12) (judge verdict: Runtime Error)

function Main(input) {
  var n = Math.floor(input);

  var ansArray = [];
  var ans = 0;
  
  while (n > 0) {
    ansArray.push(n);
    n -= 2;
  }
  
  var counts = {};
  
  for (var i = 0; i < ansArray.length; i++) {
    var key = ansArray[i];
    counts[key] = counts[key] ? counts[key] + 1 : 1;
  }
  
  for (var i = 0; i < ansArray.length; i++) {
    if (counts[5] % 1 === 1) {
      ans += counts[5] - 1;
    } else if (ansArray[i] % 10 === 0) {
      ans++;
    }
  }
  console.log(ans);
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));