#include <iostream>
#include <fstream>

using namespace std;

int five(int a)
{
    int nr5=5,nr=0;
    while(nr5<=a)
    {
        nr=nr+a/nr5;
        nr5=nr5*5;
    }
    return nr;
}


int main()
{
    ifstream f("fact.in");
    ofstream g("fact.out");
    int p,n,fives,pp,ok=1;
    f >> p;
    int st=0,dr=9,mid;
    dr=5*p;
    while(dr>st)
    {
        mid=(st+dr)/2;
        if(five(mid) > p)
        {
            dr=mid;
        }
        else if(five(mid) < p)
        {
            st=mid;
        }
        else if(five(mid) == p)
        {
            break;
        }

    }
    while(five(mid)==five(mid-1))
    {
        mid=mid-1;
    }
    g << mid;
    return 0;
}