#include<iostream>
#include<stdio.h>
using namespace std;
int main(){
    int l1,l2,p,m,c,fac,poz,i;
    freopen("factorial.in","r",stdin);
    freopen("factorial.out","w",stdout);
    cin>>p;
    l1=1;
    l2=10;
    while(l1<=l2){
        m=(l1+l2)/2;
        fac=1;
        c=0;
        for(i=1;i<=m;i++)
            fac=fac*i;
        while(fac%10==0){
            if(fac%10==0)
                c++;
            fac=fac/10;
        }
        if(c<p)
            l1=m+1;
        if(c>p)
            l2=m-1;
        if(c==p){
            poz=m;
            l1=l2+1;
        }
    }
    cout<<poz;
    return 0;
}