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: Accepted)

#include <bits/stdc++.h>
#define rep(i,cc,n) for(int i=cc;i<=n;++i)
#define drep(i,cc,n) for(int i=cc;i>=n;--i)
#define sz(s) (int)(s.size())
#define vecprint(v) rep(i,0,v.size()-1)cout << v[i] << " ";cout << endl;
#define arrprint(v,n) rep(i,0,n-1)cout << v[i] << " ";cout << endl;
#define matprint(v,n,m) rep(i,0,n-1){rep(j,0,m-1){cout << v[i][j] << " ";}cout << endl;}
#define SORT(v) sort(v.begin(), v.end())
#define REV(v) reverse(v.begin(), v.end())
using namespace std;
const int mod=1000000007;
const int INF=1001001001;
typedef long long ll;
typedef pair<int,int> P;
template<typename T>
using priority_queue_rev = priority_queue<T, vector<T>, greater<T>>; //小さいものから取り出す

int main(){
    ll x;
    cin >> x;
    if(x%2 == 1){
        cout << 0 << endl;
        return 0;
    }
    ll five = 5;
    ll ans = 0;
    while(five <= x){
        ans += x/(five*2);
        five *= 5;
    }

    cout << ans << endl;
//  printf("%.10f\n",ans);

    return 0;
}