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 C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)


// problem: "https://atcoder.jp/contests/abc148/tasks/abc148_e"

#include<bits/stdc++.h>
#define ll long long

using namespace std;

ll count_0(ll n)
{
    ll c = 0;
    ll i;

    i = 5;
    while(i <= n)
    {
        c += n/(i*2);
        i *= 5;
    }

    return c;
}

int main()
{
    ll n;
    cin>>n;

    cout<<count_0(n);       // define
    return 0;
}