#include <iostream>
#include <fstream>
#include <math.h>
#define NMax 100000001
using namespace std;
ifstream f("factorial.in");
ofstream g("factorial.out");
int ciur[NMax],p;
int main ()
{
    long long i,k=5,sum=0;
    f>>p;
    while (k<=p*5)
    {
        for (i=0;i<=p*5;i=i+k) ciur[i]++;
        k=k*5;
    }
    for (i=1;i<=p*5 && sum<p;i++)
        sum=sum+ciur[i];
    if (p==0) g<<1;
    else if (sum>p) g<<-1;
    else g<<i-1;
}