#include <iostream>
#include <cstdio>

using namespace std;

int main()
{
    freopen("fact.in","r",stdin);
    freopen("fact.out","w",stdout);
    int n,afi=0,cp,m,i=0,j,p,aux=1;
    cin>>p;
    cp=p;
    for(i=5;p>0;i+=5){
        cp=i;
        afi=0;
        while(cp%5==0){
            cp=cp/5;
                afi++;
        }
        if(p-afi<0){
            p=0;
            i=-1;
        }
        else
        p=p-afi;
    }
    cout<<i-5;
    return 0;
}