#include <fstream>
#include <cmath>
using namespace std;
unsigned long long log5(unsigned long long n)
{
    int l=0;
    while(n%5==0)
    {
        n/=5;
        l++;
    }
    return l;
}
long long powe(int x,int y)
{
    int i=0,p=1;
    for(i=1;i<=y;++i)
    {
        p*=x;
    }
    return p;
}
ifstream cin("fact.in");
ofstream cout("fact.out");
int main()
{
    unsigned long long sg,c5,p,i,c=0;
    cin>>p;
    if(p==0){
    cout<<1;
    return 0;}
    c5=log5(p);
    sg=p-(c5*(c5+1)/2);
    cout<<sg*5;
    return 0;
}