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

#include <bits/stdc++.h>
#include<string>
using namespace std;
#define ll long long
#define ld long double
#define endl '\n'
#define FAST ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
const int maxn=2*1e5+5;

int main()
{
    FAST

ll x;
cin>>x;
if(x%2==1)
    cout<<0<<endl;
x/=2;
ll res=0;
while(x)
{
    res+=x/5;
    x/=5;
}
cout<<res<<endl;

    return 0;
}