#include <iostream>
#include <fstream>
using namespace std;
long long p,nr,nnr,j;
ofstream g("fact.out");
int main()
{
    freopen("fact.in","r",stdin);
    scanf("%d",&p);
    nr=p*5;
    j=5;
    nnr=1;
    while(p>j)
    {
        nr=nr-5*nnr;
        j=j*5;
        nnr++;
    }
    if(nr==0)
        g<<1;
    else
    g<<nr;
    return 0;
}