#include <iostream>
#include <fstream>

using namespace std;

ifstream in("fact.in");
ofstream out("fact.out");

long long sum[100000001],P,S2,S=1;
const long long N=1<<50;

int caut()
{
    long long pas=N;
    int r=0;
    while(pas!=0)
    {
        if(pas+r<=5*P && sum[pas+r]<=P)
            r+=pas;
        pas/=2;
    }
    return r;
}

int main()
{
    int n=0,i=1;
    in>>P;
    sum[0]=1;
    if(P==0)
        out<<sum[P];
    else{
    while(i<=5*P)
    {
        S=S*i;
       if(S%10==0)
        {
            n++;
            S/=10;
        }
        sum[i]=n;
        i++;
    }
    out<<caut();
    }
    return 0;
}